Unable to send email in Plesk: mail transport unavailable

Symptoms

• There are lots of undelivered outgoing emails on the Plesk server. It is not possible to send mail and messages get stuck in the mail queue.

• The following errors are logged in /var/log/maillog:

  CONFIG_TEXT: postfix/smtp[715642]: connect to example.com[203.0.113.2]:587: Invalid argument
  postfix/smtp[715642]: 8A0FA1D2090C: to=john.doe@example.com, relay=none, delay=470, delays=470/0.01/0.16/0, dsn=4.4.1, status=deferred (connect to example.com[203.0.113.2]:587:
  Invalid argument)

  ---

  CONFIG_TEXT: postfix/qmgr[14111]: warning: connect to transport private/203.0.113.2-: Connection refused
  postfix/error[21724]: 0A1322CF16C: to=john.doe@example.com, relay=none, delay=0.07, delays=0.03/0.01/0/0.02, dsn=4.3.0, status=deferred (mail transport unavailable)

Cause

Postfix is misconfigured.

Resolution

Connect to the server via SSH.

If there are no custom settings in Postfix's configuration, execute plesk repair mail utility:

# plesk repair mail -y

If there are some custom settings and the issue should be resolved more accurately, verify the following:

1. Transport settings are set as below:

   # postconf -n | grep -i transport
   sender_dependent_default_transport_maps = hash:/var/spool/postfix/plesk/sdd_transport_maps
   transport_maps = , hash:/var/spool/postfix/plesk/transport
   virtual_transport = plesk_virtual

2. Transport may be defined in sender_dependent_default_transport_map if Send from domain IP addresses option is set in Tools & Settings > Mail Server Settings:

   # postmap -s /var/spool/postfix/plesk/sdd_transport_maps
   @example.com plesk-203.0.113.2-:
   @example2.com plesk-203.0.113.3-:

   Note: 203.0.113.x should be replaced with the server's IP addresses.

3. If so, the entries from sdd_transport_maps are defined in the first column of smtp_bind_address option. smtp_bind_address is an optional numerical network address that the Postfix SMTP client should bind to when making an IPv4 connection.

   # grep "smtp_bind_address" /etc/postfix/master.cf
   plesk-203.0.113.2- unix - n n - - smtp -o smtp_bind_address=203.0.113.2 -o smtp_bind_address6= -o smtp_address_preference=ipv4
   plesk-203.0.113.3- unix - n n - - smtp -o smtp_bind_address=203.0.113.3 -o smtp_bind_address6= -o smtp_address_preference=ipv4

4. To be able to connect, smtp_bind_address needs to be defined as mentioned in the mynetworks section and the binding name should be the same as defined in sdd_transport_maps:

   # postconf mynetworks
   mynetworks = 127.0.0.0/8 [::1]/128 203.0.113.2/32, 203.0.113.3/32

   Starting from Plesk 12, it should be empty:

   # postconf mynetworks
   mynetworks =

   unless there are white-listed IP addresses in Plesk > Tools & Settings > Mail Server Settings > White List tab. If so, they will be added to mynetworks section.

5. Restart the service:

   # service postfix restart