Outgoing mail does not work: local data error while talking to smtp.mailexample.com

Symptoms

• Emails are not sent from a server

• Emails are kept at Tools&Settings > Mail Server Settings > Mail Queue

• No error is shown in mail client interface.

• Ports 25 and 587 are opened.

• Trying to send an email the following messages are logged into /var/log/maillog:

  CONFIG_TEXT: example postfix/smtp[18437]: warning: hash:/etc/postfix/password is unavailable. open database /etc/postfix/password.db: No such file or directory
  example postfix/smtp[18437]: warning: hash:/etc/postfix/password lookup error for "smtp.mailexample.com"
  example postfix/smtp[18437]: warning: 61856222BB: smtp_sasl_passwd lookup error
  example postfix/smtp[18437]: 61856222BB: to=john.doe@example.com, relay=smtp.mailexample.com[203.0.113.2]:25, delay=524, delays=522/0.01/1.9/0, dsn=4.3.0, status=deferred (local data error while talking to smtp.mailexample.com[203.0.113.2])

  OR

  CONFIG_TEXT: (delivery temporarily suspended: Host or domain name not found. Name service error for name=domain.name.of.relay.server type=AAAA: Host not found)

• There is an additional section in main.cf file:

  # cat /etc/postfix/main.cf
  ..
  #MAILCHANNELS
  relayhost = smtp.mailexample.com
  smtp_sasl_auth_enable = yes
  smtp_sasl_password_maps = hash:/etc/postfix/password
  smtp_sasl_security_options = noanonymous
  smtpd_sasl_authenticated_header = yes
  smtp_tls_security_level = may
  #ENDMAILCHANNELS

• Such relayhost is not accessible:

  # telnet relay.example.tld 25
  Trying 203.0.113.2...
  telnet: connect to address 203.0.113.2: Connection timed out

Cause

Postfix misconfiguration. Manually configured relayhost is not accessible from the server.

Resolution

1. Login to the server via SSH.

2. Open /etc/postfix/main.cf with any text editor, e.g.:

   # vi /etc/postfix/main.cf

3. Comment MAILCHANNELS section:

   CONFIG_TEXT: #MAILCHANNELS
   #relayhost = smtp.mailexample.com
   #smtp_sasl_auth_enable = yes
   #smtp_sasl_password_maps = hash:/etc/postfix/password
   #smtp_sasl_security_options = noanonymous
   #smtpd_sasl_authenticated_header = yes
   #smtp_tls_security_level = may
   #ENDMAILCHANNELS

4. Restart postfix service to apply changes:

   # systemctl restart postfix