Yes: one VPS can host mail for several domains, and one Roundcube installation can provide the browser interface for all of them. The reliable design is a shared Postfix and Dovecot mail stack, virtual-mailbox data rather than separate UNIX accounts, and a single Roundcube deployment that either accepts full email addresses or selects a small host-specific configuration for each webmail hostname.
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Roundcube does not receive or deliver mail itself. It needs a functioning IMAP service, an SMTP submission service, PHP, a database and a web server. Get those layers, DNS authentication and TLS aligned first; then adding another domain is mainly a matter of adding virtual-domain, mailbox, alias and DNS records.
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What the finished architecture looks like
Use one canonical hostname, such as mail.example.net, for the server’s SMTP, IMAP and webmail endpoints. The VPS runs three cooperating services:
- Postfix accepts SMTP connections, applies recipient and alias maps, and routes mail for each hosted domain. Virtual mailbox domains keep hosted addresses separate from local UNIX accounts.
- Dovecot provides IMAP (and, if you need it, POP3), authenticates virtual users and stores their mail. Postfix commonly hands final delivery to Dovecot through LMTP.
- Roundcube is the browser interface. It logs users into Dovecot over IMAP and submits outgoing messages through authenticated SMTP.
The webmail URL can be one shared address, or you can publish a branded hostname for each domain. In the latter case, Roundcube’s host-specific configuration maps each hostname to a small configuration file. The IMAP server and its user database remain the authority for accounts and passwords; Roundcube does not create or manage users.
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Prerequisites and decisions before installing
Choose the canonical hostname
Pick one fully qualified hostname, for example mail.example.net. Create an A record (and an AAAA record only if the VPS has working IPv6) pointing to the server. Obtain a certificate covering every name users will visit, including the canonical name and any separate webmail names. Use the same canonical name in Postfix, Dovecot and Roundcube wherever possible. A consistent name avoids certificate warnings and mismatched client settings.
Confirm VPS and network prerequisites
- A supported Linux distribution, a static public IPv4 address and a provider-controlled PTR (reverse-DNS) record.
- Backups that are stored away from the VPS and a documented restore procedure.
- Provider permission for outbound TCP port 25. Some VPS providers block or restrict it; this policy must be checked before deployment.
- A firewall exposing only the services you intend to run: HTTPS (443) for webmail, SMTP 25 for server-to-server delivery, submission 587 (and optionally 465) for authenticated clients, and IMAPS 993. Open POP3 or POP3S only when there is a specific requirement.
A configuration guide may use an example such as 1 GB of RAM, but that is a setup example, not a capacity guarantee. Mailbox count, message volume, filtering and retention determine the resources you actually need.
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The most portable convention is a full address such as [email protected] as the username. If users enter only alice, Roundcube can append a domain with username_domain; that setting must be selected per webmail hostname or users with the same local part in different domains will be ambiguous. Decide this before publishing instructions.
Build the mail transport and mailbox layers
Install and align Postfix and Dovecot
Install Postfix for SMTP and Dovecot for IMAP/POP3 and authentication using your distribution’s supported packages. Configure TLS certificates before exposing authenticated services. Dovecot should use a virtual-user database (SQL or another supported backend) so accounts do not require UNIX login accounts. Keep the following values consistent across both services:
- Virtual-user email addresses and password hashes.
- Mailbox location and the UID/GID that owns the mail files.
- Quota policy and namespace settings.
- The SASL authentication socket or service that Postfix uses for SMTP submission.
Require TLS before allowing authentication. Disable plaintext authentication on unencrypted connections. Test an IMAP login and an SMTP submission login with the same account before adding more domains.
Declare hosted domains in Postfix
For domains that have mailboxes on this VPS, use Postfix’s virtual mailbox features. A virtual mailbox domain provides addresses without creating operating-system users. The essential maps are:
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virtual_mailbox_domains— the domains for which this server is an authoritative mailbox host.virtual_mailbox_maps— complete recipient addresses mapped to a mailbox path or a Dovecot delivery target.virtual_alias_maps— aliases, forwarding addresses and role accounts.- A Dovecot LMTP transport, when Dovecot performs final delivery.
An address map should contain complete addresses, not only local parts. Conceptually, entries look like this:
[email protected] example.com/alice/
[email protected] example.com/billing/
[email protected] example.org/admin/The exact right-hand side depends on your chosen Dovecot mailbox layout and delivery transport. Do not copy a path blindly: it must match Dovecot’s mail_location, ownership and namespace configuration.
Aliases are separate from real mailboxes. For example, [email protected] can deliver to [email protected] without creating a second mailbox. Forwarding addresses should be controlled carefully because unrestricted forwarding can create abuse and backscatter.
Keep local domains separate from hosted domains
List the VPS hostname and other machine-local names in Postfix’s local-domain settings, and keep hosted customer domains in the virtual-domain map. Postfix warns that all machine hostnames must be accounted for; omitting one can cause mail loops or unexpected local delivery. Do not add a hosted domain to both local and virtual delivery classes.
Configure Dovecot storage and authentication
Choose a mailbox layout
Use a deterministic layout that includes the domain, such as domain/user/. Including the domain prevents collisions when both [email protected] and [email protected] exist. The mailbox path used by Dovecot, the path returned by the Postfix mailbox map and the filesystem ownership must agree exactly.
Use one authoritative credential store
Dovecot should authenticate virtual users from one database or supported user backend. Store passwords with a modern, salted hash supported by your Dovecot version. Configure Postfix SMTP AUTH to use Dovecot’s authentication service rather than maintaining a second password list. This makes a password change effective for both IMAP and SMTP submission.
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Deliver through LMTP when appropriate
LMTP lets Postfix hand a message to Dovecot for final mailbox delivery, where Dovecot can apply namespaces, quotas and delivery rules consistently. If you use LMTP, verify that the LMTP socket is reachable by Postfix and that the virtual user exists before accepting mail. A successful SMTP acceptance followed by a failed local delivery indicates a map, socket, permission or quota problem rather than a DNS problem.
Install Roundcube once
Meet Roundcube’s application requirements
Roundcube requires an HTTP server, PHP, a database, an IMAP server and an SMTP server. Install it under the web server, create its database, and point the site’s document root at Roundcube’s public_html directory. Follow the version’s supported PHP and database requirements rather than assuming that the operating system’s newest release is compatible.
The installer and runtime need writable temp and logs directories. Grant only the web-server account the necessary write permission; do not make the entire installation world-writable. After installation, remove or protect any setup endpoint that your package leaves enabled.
Set the default IMAP and SMTP endpoints
Configure Roundcube to use the canonical mail hostname for IMAP and authenticated SMTP. Use IMAPS on 993 and submission on 587 (or the separately configured secure submission service). The certificate name presented by those services must match the hostname Roundcube uses. If Roundcube can display the login page but reports an IMAP connection error after submission, check the certificate name, firewall and Dovecot listener before changing application settings.
Serve several domains from the one Roundcube installation
One shared URL with full-address logins
If every user visits one URL and enters a complete address, one default Roundcube configuration is often sufficient. All domains can share the same IMAP host and SMTP submission host. This is the simplest operational model: add the domain to Postfix, Dovecot and DNS, then create its virtual users and aliases.
Separate webmail hostnames with host-specific files
Use host-specific configuration when you want addresses such as webmail.example.com and webmail.example.org, different login defaults, or different IMAP backends. Roundcube supports a host-to-file map through include_host_config. The following illustrates the pattern:
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$config['include_host_config'] = array(
'webmail.example.com' => 'example_config.inc.php',
'webmail.example.org' => 'org_config.inc.php',
);Place the referenced files where your Roundcube version expects host configuration files. A domain-specific file can set the IMAP host and append the domain when a user enters only the local part:
$config['imap_host'] = 'ssl://mail.example.net:993';
$config['smtp_server'] = 'tls://mail.example.net:587';
$config['username_domain'] = 'example.com';Use the second file with username_domain set to example.org. If users enter full addresses, omit username_domain to avoid appending a domain twice. Keep SMTP submission on the same authenticated identity that Dovecot accepts over IMAP.
Certificate and virtual-host details
Configure the HTTP server with a virtual host for every published webmail name and redirect HTTP to HTTPS. The certificate must contain each name, and DNS for each name must point to the VPS. A correct Roundcube host map cannot compensate for a web-server virtual host that sends the request to another site or for a certificate that omits the requested name.
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Repeat the following records for each domain you host. Values shown are roles; use the actual server name, selector and policy you have configured.
| Record | Purpose | What to verify |
|---|---|---|
| MX | Routes inbound mail | Points to the canonical mail hostname, not directly to an IP address; the hostname has working A/AAAA records. |
| SPF (TXT) | Authorizes sending hosts | Includes the VPS’s real outbound sender and does not exceed DNS lookup limits after all includes are expanded. |
| DKIM (TXT) | Publishes the public key for signed mail | The selector and key exactly match the signer used by your outbound MTA. |
| DMARC (TXT at _dmarc) | Defines receiver policy and reporting | Alignment rules match your From domain and the policy is introduced deliberately. |
| PTR (reverse DNS) | Gives the VPS IP a stable name | The provider’s PTR points to the canonical hostname, which in turn resolves back to the IP where practical. |
MX records decide where a domain receives mail; SPF, DKIM and DMARC influence whether other providers trust messages sent from it. They do not replace one another. A domain can receive mail correctly while its outbound mail is rejected because its sender authentication or reverse DNS is missing.
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Encrypt every user-facing path
- Serve Roundcube only over HTTPS.
- Use IMAPS on 993 or STARTTLS on a deliberately configured alternative; do not allow credentials over cleartext IMAP.
- Use authenticated submission on 587 (and 465 only when intentionally configured) rather than asking users to send through port 25.
- Renew certificates before expiry and reload Postfix, Dovecot and the web server after renewal when required.
Control abuse and resource consumption
Apply connection and authentication rate limits, fail2ban or an equivalent control, mailbox quotas, and spam and malware filtering. Monitor failed logins, queue growth, disk usage, deferred deliveries and certificate expiry. Keep the operating system, Postfix, Dovecot, Roundcube, PHP and filtering components updated from supported packages.
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Back up what is needed to rebuild
Back up mailbox data, the virtual-user database, alias and domain maps, DKIM private keys, Roundcube configuration, its database and TLS material according to your security policy. Test restoring to a separate environment. A backup that contains messages but not the user database or keys is not a complete mail-service recovery.
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Adding a second or later domain
- Create the domain’s A or AAAA records for any webmail hostname and add the MX, SPF, DKIM and DMARC records.
- Add the domain to Postfix’s virtual mailbox-domain map. Ensure it is not also classified as a local domain.
- Create virtual mailbox entries and aliases, including required role addresses such as postmaster.
- Add matching virtual users, mailbox locations, quotas and password hashes to Dovecot’s backend.
- If the domain has its own webmail hostname, add a Roundcube host-map entry and a small configuration file. Set
username_domainonly when local-part-only logins are intended. - Reload or gracefully restart the affected services according to your distribution’s service manager.
- Test one inbound message, one authenticated outbound message, IMAP access and HTTPS access for the new domain before announcing it.
Testing checklist before production
- Resolve the MX and webmail names from an external network.
- Confirm the certificate presented by HTTPS, IMAP and SMTP submission contains the name each client uses.
- Log into IMAP with a full address and verify the expected mailbox and folders.
- Authenticate to SMTP submission with the same credentials and send to an external test address.
- Send an external message to each hosted domain and confirm that Postfix maps the recipient to the intended Dovecot mailbox.
- Test an alias and verify that an unknown recipient is rejected rather than silently accepted.
- Inspect DKIM signing, SPF evaluation and DMARC alignment in the receiving system’s headers.
- Check queue, authentication and delivery logs for errors, repeated retries and unexpected relay attempts.
- Verify quotas, backup completion and monitoring alerts.
These checks are operational validation steps, not a substitute for a live mail-deliverability test from several receiving providers. There is no universal performance figure for a VPS mail stack; capacity depends on workload and the controls you enable.
Troubleshooting by symptom
Mail for one domain never arrives
Check that the domain’s MX points to the canonical hostname, that the hostname resolves publicly, and that the domain appears in virtual_mailbox_domains. Then verify the complete recipient address in virtual_mailbox_maps, the Dovecot user record and the LMTP delivery path. If Postfix accepts the message but it remains deferred, inspect the delivery and quota errors rather than changing DNS.
Roundcube says the password is wrong
Try the same address and password with a direct IMAP client. If direct IMAP fails, correct Dovecot’s user database, password hash or TLS settings. If direct IMAP succeeds, inspect the Roundcube host mapping: an incorrect username_domain can turn alice into the wrong address, and an incorrect imap_host can send the login to another server.
Outgoing mail is rejected or marked suspicious
Confirm that the VPS PTR, SMTP banner and TLS certificate use the canonical hostname. Check SPF authorization, DKIM signing and DMARC alignment for the exact From domain. Also verify that the provider permits outbound port 25 and that you are sending through authenticated submission rather than an unintended relay path.
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One webmail hostname shows the wrong settings
Check the HTTP virtual host, DNS destination and the exact hostname key in include_host_config. Host matching is literal: a typo, alternate name or unexpected trailing label can cause Roundcube to load its default configuration. Ensure each referenced file is readable by the web-server account and contains only the settings intended for that domain.
Users with the same local part collide
Use full-address usernames and a mailbox layout that includes the domain. Do not rely on local-part-only logins when both [email protected] and [email protected] exist unless each hostname has an explicit and correct username_domain.
Choosing between a hand-built stack and an alternative
A hand-built Postfix, Dovecot and Roundcube stack gives precise control, but you own every update and operational task. Before choosing a control panel or managed mail service, compare the following capabilities rather than comparing only installation time:
| Decision area | Questions to ask |
|---|---|
| Administration | Can you add domains, mailboxes, aliases and quotas without editing several unrelated systems? |
| Identity storage | Is there one authoritative virtual-user database, and can it be exported for migration? |
| TLS | Are certificates issued, renewed and applied to HTTPS, IMAP and SMTP consistently? |
| Authentication DNS | Does the workflow support SPF, DKIM, DMARC and reverse-DNS coordination for every domain? |
| Filtering and abuse | What spam, malware, rate-limit and incident-response controls are included? |
| Backup and restore | Can messages, credentials, keys, configuration and databases be restored together? |
| Scaling | How do storage, CPU, queue volume and mailbox count affect cost and performance? |
| Provider policy | Is outbound port 25 available, and what support exists for abuse complaints and blocklists? |
| Auditability | Are SMTP, IMAP, administration and delivery logs retained long enough to investigate incidents? |
The one-installation model is technically sound when these responsibilities are explicit. It is not a shortcut around deliverability, security or maintenance: every hosted domain adds DNS records, identities, aliases, keys and operational decisions.
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