Configure Sendmail as a smart-host client for smtp.gmail.com. Use port 587 with STARTTLS in most installations, or port 465 when your Sendmail build supports implicit TLS. Gmail requires SMTP authentication: use the complete Gmail or Google Workspace address with a dedicated app password after enabling 2-Step Verification, or use OAuth 2.0 when your Sendmail integration supports the required SASL mechanism.
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For organization-wide printers, servers, and applications, Google recommends its SMTP relay service rather than routing every device through one mailbox. The direct Gmail service is documented at Google Workspace Admin Help and in Gmail protocol documentation.
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What you need before changing Sendmail
This procedure assumes that Sendmail is already installed, can deliver local mail, and is built with SMTP AUTH and TLS support. You also need administrative access to its configuration and mail-log files. The exact paths and service commands differ between Linux and Unix distributions, so use your operating system’s Sendmail packaging conventions rather than copying a service command from another platform.
- A Gmail or Google Workspace mailbox that is permitted to send through Gmail SMTP.
- Outbound DNS resolution and TCP access from the Sendmail host to the selected SMTP port.
- A CA certificate bundle on the host so Sendmail can verify Google’s certificate during STARTTLS.
- Permission to edit the platform’s Sendmail
.mcsource and generate the correspondingsendmail.cf. - A protected location for the SMTP authentication map and its generated database.
Do not put a normal Google account password in a world-readable file or in source control. Credentials in an authentication map are operational secrets and should be readable only by the account that runs Sendmail and the administrators who maintain it.
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Choose the Gmail transport and relay scope
Google maps its direct SMTP service to two common encrypted transports. Port 587 starts as ordinary SMTP, upgrades the connection with STARTTLS, verifies the peer certificate, and then authenticates. Port 465 negotiates TLS before any SMTP commands and is therefore an implicit-TLS connection. The current port guidance is documented by Google and Gmail’s protocol documentation.
| Use case | Hostname | Port | Encryption sequence | Authentication |
|---|---|---|---|---|
| Single mailbox or small Sendmail host | smtp.gmail.com |
587 | SMTP connection, STARTTLS, certificate verification, then SMTP AUTH | Full address plus app password, or supported OAuth 2.0 |
| Single mailbox or small Sendmail host with implicit TLS | smtp.gmail.com |
465 | TLS is negotiated before SMTP commands | Full address plus app password, or supported OAuth 2.0 |
| Organization devices and applications | smtp-relay.gmail.com |
25, 465, or 587 | Depends on the selected port and the relay policy | Configured by the Workspace relay service; commonly scoped by organization policy or source IP rather than one mailbox password |
Google explicitly recommends the SMTP relay service for devices and applications. Choose direct Gmail SMTP when Sendmail represents one mailbox or a small, clearly owned sending identity. Choose the Workspace relay when many devices or applications must send under an organization-managed policy, when rotating one mailbox credential would be impractical, or when you need centralized relay controls.
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Use an app password for a supported Sendmail deployment
- Sign in to the Google account that Sendmail will use.
- Enable 2-Step Verification if the account does not already require it.
- Create an app password dedicated to this Sendmail host. Label it with the host or service name so it can be revoked without disturbing unrelated applications.
- Record the generated app password in your secret-management process. Treat it as a password, not as configuration text that can be shared in tickets or committed to a repository.
- Use the complete mailbox address as the SMTP login. An address at a custom Google Workspace domain is not interchangeable with a shortened local username.
Google documents app-password requirements and Gmail protocol authentication in its Workspace SMTP guidance and protocol documentation. If the account’s policies do not permit app passwords, do not substitute the normal account password; use the approved OAuth 2.0 flow or an organizational relay.
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Use OAuth 2.0 when the integration supports it
Gmail protocols support OAuth 2.0. Sendmail itself must have a compatible SASL implementation and a way to obtain, refresh, and securely store OAuth tokens; availability depends on the installed Sendmail and SASL components. Confirm the exact mechanism exposed by your build, such as an XOAUTH2 or OAUTHBEARER integration, before selecting this route. OAuth is not a drop-in replacement for an app-password string: it requires registering the client, obtaining consent and tokens, and arranging token refresh without placing a long-lived password in the authinfo map.
Point Sendmail at Gmail with SMART_HOST
Sendmail’s SMART_HOST setting routes outbound mail to a relay instead of having the local daemon deliver directly to each recipient domain. Edit the platform’s .mc source and add the Gmail hostname:
define(`SMART_HOST', `smtp.gmail.com')dnlThe exact location of the .mc file and the command used to generate sendmail.cf vary by operating system. Keep the change in the source .mc file; editing the generated .cf directly will be overwritten the next time configuration is rebuilt. If you choose the organization relay, substitute smtp-relay.gmail.com and apply the relay service’s policy requirements instead of a mailbox-specific setup.
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Enable SMTP AUTH with authinfo
Sendmail’s authinfo feature supplies SMTP AUTH credentials to the relay. Enable it in the supported form for your installed version, and generate a protected map database. A common m4 declaration is:
FEATURE(`authinfo', `hash -o /etc/mail/authinfo.db')dnlThe map syntax, source-file location, ownership, and permissions are distribution-specific. Read the Sendmail cf documentation shipped with your installation before creating the entry. The important matching rule is exact: the authinfo host key must be smtp.gmail.com when SMART_HOST is smtp.gmail.com. A key for an alias, a short hostname, or a different relay name will not be selected for the Gmail connection.
Store the complete Gmail address in the username field and the generated app password in the password field, or configure the OAuth mechanism supported by your deployment. Sendmail warns that plaintext mechanisms such as PLAIN and LOGIN should not be used unless a strong encryption layer such as STARTTLS or an external SSL tunnel is active. Port 587 therefore requires TLS negotiation to succeed before those mechanisms are allowed.
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- Restrict the authinfo source and database to the Sendmail service account and administrators.
- Keep the source file out of public backups, paste services, and source repositories.
- After rotating a credential, rebuild the map and reload Sendmail; an old generated database can continue to return the revoked secret.
- Check that the relay hostname in the map and the value of
SMART_HOSTare identical, including spelling.
Configure certificate trust and TLS
STARTTLS is not only an encryption switch; Sendmail must also authenticate the server certificate. Install the CA bundle maintained by the host operating system and point Sendmail’s TLS configuration to it with the appropriate confCACERT file and/or confCACERT_PATH hashed directory. Sendmail’s STARTTLS support requires these trust settings to verify the peer certificate.
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For port 465, ensure the installed Sendmail/SASL stack actually supports implicit TLS. Some builds expose STARTTLS settings but do not provide a client mode for TLS-before-SMTP. If implicit TLS is unavailable, use port 587 with STARTTLS or deploy the external TLS method supported by your platform rather than sending unencrypted commands.
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Build, reload, and perform a controlled test
- Save the
.mcsource, the authinfo source, and the TLS CA settings with restrictive permissions. - Regenerate
sendmail.cfusing the operating system’s supported m4/build process. Do not assume that a command from another distribution uses the same directory or wrapper. - Confirm that the generated configuration contains the intended smart host and authinfo feature, without printing the secret into a shared terminal transcript.
- Protect the generated configuration and authinfo database, then reload or restart the Sendmail service using the host’s service manager.
- Send one test message to an address you control. Use a unique subject and timestamp so it can be correlated with the mail log.
- Inspect the mail log while the test runs. Look for DNS resolution, TCP connection, STARTTLS negotiation, certificate verification, SMTP AUTH response, Gmail policy response, and final queue status.
- Repeat with a recipient at a different domain only after the first authenticated submission succeeds. This separates Gmail submission problems from recipient-domain delivery or spam filtering.
Service names, reload commands, and log locations are not uniform across Linux and Unix. If the test message remains queued, inspect the queue and the first SMTP error rather than repeatedly retrying; repeated retries can obscure the original failure and may compound throttling.
Troubleshooting Sendmail-to-Gmail failures
| Symptom | Likely cause | What to check or change |
|---|---|---|
| Authentication rejected | A normal account password was used, the app password is invalid or revoked, 2-Step Verification or an app-password policy is not satisfied, or the OAuth SASL flow is mismatched. | Create a new app password after confirming the account policy, use the full account address, or validate the OAuth token and mechanism. Do not keep retrying a known-invalid password. |
| STARTTLS or certificate error | Missing CA bundle, incorrect confCACERT/confCACERT_PATH, unreadable trust directory, expired bundle, or a Sendmail build without client TLS support. |
Verify the configured path and permissions, refresh the operating-system CA bundle, confirm Sendmail TLS support, and inspect the log for the certificate name or chain that failed. |
| Relay denied or authentication appears unused | The authinfo key does not exactly match smtp.gmail.com, the map database was not rebuilt, or SMART_HOST points to a different host than expected. |
Compare the generated smart-host value and map key character-for-character, rebuild the map, reload Sendmail, and check that the intended feature is present in the generated configuration. |
| Connection timeout or connection refused | An egress firewall, security group, proxy, or ISP policy blocks the selected SMTP port, or DNS cannot resolve the relay. | Test DNS resolution and outbound TCP access to port 587 or 465 from the Sendmail host. If one permitted port is blocked, use the other supported encrypted transport after confirming the Sendmail TLS mode. |
| Messages are throttled or rejected after successful login | Gmail sending limits, spam or policy enforcement, or an unsuitable mailbox-as-relay design. | Review the documented Gmail SMTP limit of 2,000 messages per day, inspect the exact SMTP response, reduce or queue traffic, and move organization-scale device/application traffic to the Workspace SMTP relay. |
| Port 465 connects but SMTP commands fail | The client opened a plain SMTP session to a port expecting implicit TLS, or the local Sendmail build cannot act as an implicit-TLS client. | Use the platform’s documented implicit-TLS configuration, or switch to port 587 and require STARTTLS. |
Direct Gmail SMTP versus Workspace SMTP relay
Both services can submit mail through Google, but they solve different operational problems. Direct Gmail SMTP associates every submission with one mailbox credential. The Workspace relay is intended for devices and applications and is administered as an organizational service.
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| Decision factor | smtp.gmail.com |
smtp-relay.gmail.com |
|---|---|---|
| Primary scope | One Gmail or Workspace mailbox used by Sendmail | Organization-managed printers, servers, and applications |
| Credential model | Full mailbox address plus app password, or supported OAuth 2.0 | Workspace relay policy, commonly scoped by approved source IP or organizational settings |
| Documented ports | 465 for SSL/implicit TLS and 587 for TLS/STARTTLS | 25, 465, and 587 |
| Operational ownership | Mailbox owner or small service team rotates the credential | Workspace administrators manage relay access and policy |
| Best fit | A limited number of trusted Sendmail hosts representing a known sender | Many devices or applications that should not share one mailbox password |
Google’s guidance recommends SMTP relay for sending from devices or applications. A relay does not remove the need to define encryption, source restrictions, and logging; it changes where those controls are administered.
Security and maintenance checklist
- Use port 587 with verified STARTTLS unless your tested Sendmail build specifically requires implicit TLS on port 465.
- Never use a normal Google account password in Sendmail. Prefer a dedicated app password where policy permits, or a fully implemented OAuth 2.0 integration.
- Keep the authinfo source and database out of world-readable locations and repositories.
- Match the authinfo key to the exact smart-host hostname.
- Maintain the operating system’s CA bundle and test certificate verification after CA or Sendmail upgrades.
- Monitor mail logs for authentication failures, TLS errors, queue growth, Gmail policy responses, and quota-related responses.
- Rotate app passwords when staff, hosts, or ownership changes, then rebuild the map and reload Sendmail.
- For growing or multi-device workloads, plan a move to the Workspace SMTP relay before a mailbox limit or policy block interrupts delivery.
What a successful submission looks like
A correctly configured port-587 flow resolves smtp.gmail.com, opens a TCP connection, receives the server greeting, negotiates STARTTLS, validates Google’s certificate against the configured CA trust, authenticates with the complete address and approved credential, accepts the message for submission, and records a successful queue or delivery result in the Sendmail log. A port-465 flow performs the TLS handshake before the SMTP greeting and then follows the same authentication and submission stages.
If any stage fails, use the first corresponding log entry to choose the fix: DNS or TCP errors indicate network access, TLS errors indicate encryption or trust configuration, authentication errors indicate credentials or SASL, and policy or quota responses indicate that the chosen Gmail service is not appropriate for the traffic pattern.
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