Linux Mail Server

Linux Mail Server

Distro support

Postfix and Dovecot run on both families and their config layout (/etc/postfix/main.cf, /etc/dovecot/) is largely identical; postconf, postmap, postqueue, newaliases behave the same. The differences are a few package names, the mail log path, and SELinux. Body uses Debian/Ubuntu; the RHEL family (Fedora, RHEL, CentOS Stream, Rocky, Alma, Oracle) equivalents are in the matrix.

RHEL-family gotchas: opendkim (and some milters) come from EPEL (ensure_epel) on RHEL/Rocky/Alma, but ship in main on Fedora. SELinux governs Postfix/Dovecot — non-default mail-spool or maildir paths need the right contexts (restorecon, semanage fcontext) and some integrations need booleans. Mail logs are in /var/log/maillog (cross-ref linux-log-management). Deep SELinux coverage is in linux-server-hardening / Phase 2.

In sk-* scripts use the common.sh primitives (pkg_install, ensure_epel, svc_name) instead of hardcoding the family. See linux-bash-scripting and docs/multi-distro/plan.md.

Use when

• Managing Postfix, Exim, Dovecot, queue behavior, or SMTP testing.
• Debugging delivery failures, spam-folder placement, or TLS/authentication issues.
• Updating SPF, DKIM, or DMARC-related server-side behavior.

Do not use when

• The task is general DNS hosting unrelated to mail service; use linux-dns-server.
• The task is generic network reachability without a mail-specific angle; use linux-network-admin.

Required inputs

Workflow

1. Identify whether the issue is configuration, queueing, transport, or authentication.
2. Inspect current config, queue state, and relevant logs.
3. Apply the matching workflow below for reputation, submission, queue, or domain changes.
4. Re-test SMTP flow and authentication to confirm the result.
5. Stop if relay scope, DNS/TLS ownership, queue impact, test-recipient authority, or rollback is unresolved.
6. Recover by restoring the prior validated config/records, reloading the service, and retesting a controlled message before resuming the queue.

Quality standards

• Verify with real SMTP tests and queue inspection, not assumption.
• Keep authentication records and mail config aligned.
• Separate transport problems from reputation and policy problems.

Anti-patterns

• Treating spam placement as only connectivity. Fix: inspect SPF, DKIM, DMARC, reverse DNS, TLS, reputation, and message evidence separately.
• Flushing/deleting a queue before diagnosis. Fix: sample queue IDs, reasons, age, destination patterns, and logs first.
• Enabling an open relay or broad trusted network. Fix: restrict relay to authenticated/explicit clients and test rejection externally.
• Publishing mail-authentication records without alignment tests. Fix: validate selectors, policy, envelope/header domains, and authoritative DNS.
• Logging or sharing message bodies/credentials unnecessarily. Fix: use queue IDs, headers, and redacted protocol traces.

Outputs

References

• references/postfix-reference.md
• references/email-authentication.md
• references/debugging-delivery.md

Evidence Produced

Capability contract

Diagnosis defaults to read-only. Mail configuration, queue mutation, DNS publication, certificate changes, service reloads, or sending external test mail require explicit authority. Never expose mailbox credentials, private keys, or message content beyond approved evidence.

Degraded mode

Without host, DNS, or recipient access, assess only available layers and label the rest not assessed. A successful TCP connection is not a delivery or reputation pass.

Decision rules

Worked example

For a growing Postfix queue to one provider, sample deferred queue IDs, correlate enhanced status codes and logs, verify DNS/TLS/authentication, correct the evidenced cause, reload only after config validation, retry a small sample, and record delivery before releasing the remainder.

This skill is self-contained. Every command below is a standard mail tool (postfix, postqueue, postconf, swaks, openssl, dig) present on both Debian/Ubuntu and the RHEL family; body examples use Debian/Ubuntu syntax, with RHEL-family differences in the Distro support matrix above. The sk-* scripts in the Optional fast path section are convenience wrappers — never required.

This skill covers running and debugging mail on a Debian/Ubuntu or RHEL-family server: Postfix (default), Exim (alternative), Dovecot for IMAP, and the three pillars of email authentication — SPF, DKIM, DMARC.

It does not own:

• Firewall rules for SMTP ports — linux-firewall-ssl.
• DNS records themselves (MX, SPF, DKIM, DMARC live at the DNS host) — but this skill validates them.
• Mail clients — out of scope.

Informed by Linux Network Administrator's Guide (Sendmail/Exim chapters, translated to Postfix) and modern email authentication practice.

When to use

• Setting up a server to send mail (transactional, notifications, relay).
• Diagnosing "my email goes to spam."
• Checking a mail queue for stuck messages.
• Testing SMTP from the command line (EHLO, STARTTLS, auth, RCPT).
• Validating SPF, DKIM, DMARC records for a domain.
• Inspecting a remote MX's reachability and TLS.

When NOT to use

• Setting up cloud email (SES, SendGrid, Postmark) — those are API-based.
• Managing DNS records at a registrar — use the registrar's UI or API.

Standing rules

1. Never run a mail server without SPF + DKIM + DMARC. Unauthenticated mail is treated as spam by every modern receiver.
2. Never relay mail for third parties without authentication. Open relays are abuse vectors. mynetworks in main.cf must be 127.0.0.0/8 only by default.
3. Always enable TLS for submission (port 587). STARTTLS only, reject plain auth.
4. Always validate config before reload. postfix check for Postfix, exim -bV for Exim.
5. The queue is the first thing to check on delivery problems.
6. Log everything to /var/log/mail.log (syslog) and journalctl -u postfix — both exist on modern Ubuntu.
7. Reverse DNS (PTR) of the sending IP must match the HELO name. Mismatched PTR is the single biggest reason mail is flagged as spam.

Quick reference — manual commands

Postfix config and status

# Validate config — always run before reload
sudo postfix check

# Show effective config (difference from default)
postconf -n

# Reload
sudo postfix reload

# Or full restart if service behavior is stuck:
sudo systemctl restart postfix

# Service status
sudo systemctl status postfix --no-pager
sudo journalctl -u postfix -n 50 --no-pager
sudo tail -f /var/log/mail.log

Queue inspection

# Show queue
sudo postqueue -p
sudo mailq                                       # alias

# Count by status
sudo postqueue -p | grep -c '^[A-F0-9]'

# Inspect a specific message
sudo postcat -q <queue-id>

# Force a flush (try delivery now)
sudo postqueue -f

# Delete a specific stuck message
sudo postsuper -d <queue-id>

# Delete everything (use with extreme care)
sudo postsuper -d ALL

SMTP testing from the command line

# Install swaks if needed
sudo apt install swaks

# Full SMTP conversation with TLS and auth
swaks --server mail.example.com \
      --port 587 \
      --from sender@example.com \
      --to recipient@example.org \
      --auth LOGIN \
      --auth-user sender@example.com \
      --tls \
      --header "Subject: test $(date)" \
      --body "test body"

# Just the handshake (no auth, no send)
swaks --server mail.example.com --port 587 --quit-after STARTTLS

# With openssl (lower level):
openssl s_client -starttls smtp -connect mail.example.com:587 -crlf
# Type: EHLO test.example.com
# Then: QUIT

Email authentication checks

# SPF lookup
dig +short TXT example.com | grep spf1

# DKIM selector (replace "default" with your actual selector)
dig +short TXT default._domainkey.example.com

# DMARC
dig +short TXT _dmarc.example.com

# MX record with priorities
dig +short MX example.com

# Reverse DNS (PTR) — must match HELO
dig +short -x $(curl -s https://api.ipify.org)

# Online checkers (for humans, but record the dig output yourself first):
# - https://mxtoolbox.com
# - Gmail Postmaster Tools

Full email-auth deep dive (record syntax, DKIM key generation via opendkim-genkey, DMARC reporting, example DNS zone snippet) — see references/email-authentication.md.

Typical workflows

Workflow: "Our emails are going to spam"

Walk references/debugging-delivery.md. Condensed:

# 1. Queue health
sudo postqueue -p

# 2. Reject reason on a deferred message
sudo postcat -q <queue-id> | tail -20

# 3. SPF / DKIM / DMARC present?
dig +short TXT example.com | grep spf1
dig +short TXT default._domainkey.example.com
dig +short TXT _dmarc.example.com

# 4. PTR matches HELO?
dig +short -x $(postconf -h inet_interfaces | awk '{print $1}')
postconf -h myhostname

# 5. TLS working for submission?
swaks --server mail.example.com --port 587 --quit-after STARTTLS

Workflow: "Is port 587 submission working?"

swaks --server mail.example.com \
      --port 587 \
      --auth LOGIN \
      --auth-user bot@example.com \
      --tls

Reports each step (connection, STARTTLS, AUTH, MAIL FROM, RCPT TO, DATA).

Workflow: "The queue is growing"

# 1. How bad?
sudo postqueue -p | tail -5   # last line says "Total requests"

# 2. What's stuck and why?
sudo postqueue -p | head -30

# 3. Pick a message and see the specific rejection:
sudo postcat -q <queue-id>

# 4. Common fixes:
#    - Greylist: just wait; Postfix will retry automatically
#    - TLS handshake error: check recipient server's cert, your own
#    - Auth required: fix relay credentials
#    - Relay denied: check mynetworks, smtpd_recipient_restrictions

Workflow: "Add a new domain to an existing Postfix"

# 1. Edit main.cf — add to mydestination or virtual_mailbox_domains
sudo nano /etc/postfix/main.cf

# 2. If using virtual mailboxes, update the virtual maps:
sudo nano /etc/postfix/virtual_alias_maps
sudo postmap /etc/postfix/virtual_alias_maps

# 3. Validate
sudo postfix check

# 4. Reload
sudo postfix reload

# 5. Test with swaks

Troubleshooting / gotchas

• PTR record mismatch is the #1 spam trigger. If your server's IP reverse-resolves to vps-12345.provider.net but Postfix HELOs as mail.example.com, receivers downgrade you immediately. Fix the PTR at the VPS provider.
• postfix reload doesn't reload everything. Changes to master.cf require sudo systemctl restart postfix. postfix reload only re-reads main.cf.
• TLS fails on port 465 but works on 587. Port 465 is implicit TLS (SMTPS), 587 is STARTTLS. They need separate service stanzas in master.cf — enable both.
• DKIM signs but receivers fail DMARC. Check alignment: the signing domain (d=) must match the From: header domain (relaxed alignment) or exactly (strict). A DKIM-signed bounce that uses the mail server's hostname as d= won't align with the sender's domain.
• Postfix is silently deferring. Look in /var/log/mail.log — the rejection reason is there. The queue only shows the summary.
• Dovecot and Postfix disagree about a user. Postfix's virtual_mailbox_maps must match Dovecot's passdb / userdb. Use a single source of truth (MySQL, LDAP, or a flat file) and point both at it.

References

• references/postfix-reference.md — full Postfix reference: main.cf parameters, master.cf, queue management, 3 complete config examples.
• references/email-authentication.md — SPF, DKIM (with opendkim), DMARC deep dive with DNS record examples.
• references/debugging-delivery.md — decision tree for delivery problems.
• Book: Linux Network Administrator's Guide (Kirch & Dawson) — mail chapters (Sendmail/Exim, translated to Postfix).
• Book: Ubuntu Server Guide (Canonical) — Postfix and Dovecot.
• Man pages: postfix(1), postconf(5), master(5), postqueue(1), postcat(1), swaks(1).

Optional fast path (when sk-* scripts are installed)

Running sudo install-skills-bin linux-mail-server installs:

These are optional wrappers around dig, swaks, postqueue, and openssl s_client.

Scripts

This skill installs the following scripts to /usr/local/bin/. To install:

sudo install-skills-bin linux-mail-server