Email is delivered with a delay

Symptoms

• Email messages and mailing list deliveries are delayed

• Greylisting is enabled

• One of the following errors might appear in /var/log/maillog file:

  handlers_stderr: DEFER
  DEFER during call 'grey' handler
  Message aborted.
  milter-reject: DATA from mail.example.com[203.0.113.2]: 451 4.7.1 Service unavailable - try again later; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<mail.example.com>

  greylisting filter[10489]: Starting greylisting filter...
  /usr/lib64/plesk-9.0/psa-pc-remote[8641]: handlers_stderr: REJECT
  /usr/lib64/plesk-9.0/psa-pc-remote[8641]: REJECT during call 'grey' handler
  /usr/lib64/plesk-9.0/psa-pc-remote[8641]: Message aborted.
  postfix/smtpd[10450]: reply: SMFIR_REJECT data 0 bytes
  postfix/smtpd[10450]: E612613C0057: milter-reject: DATA from unknown[203.0.113.2]: 550 5.7.1 Command rejected; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<mail.example.com>

• Mailing list deliveries are delayed and the following error is shown in /var/log/maillog file:

  postfix/smtpd[1598278]: 85AB012F841: milter-reject: DATA from localhost[127.0.0.1]: 451 4.7.1 Service unavailable - try again later; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<mail.example.com>

  And the following error is shown in /var/log/mailman/smtp-failure file:

  (2430) delivery to [email protected] failed with code 451: 4.7.1 Service unavailable - try again later

Cause

This is expected the behavior of Greylisting.

Greylisting works as follows: the first message is rejected, and the next message sent from the same address (sender server IP address and 'From:') will be accepted after a certain length of time passes.

After the first email is rejected, the sender's address is added to the Greylisting database. The information is stored there for an expiration time interval.
For the message to be accepted, the grey interval must complete. If the message was sent before the grey interval has passed, the penalty interval is added.

Resolution

To prevent a delay, perform one of the following actions:

• Reduce the grey interval according to the following KB article: How to configure Greylisting?

• Go to Plesk > Tools & Settings > Spam Filter Settings > White List and add the required mailbox, domain or pattern to the white-listed senders and their emails will be accepted without passing through the greylisting and SpamAssassin.

  • Adding an email address to the whitelist: [email protected]

  • Adding all emails sent from a specific domain to the whitelist: *@example.com

  Note: In case an email sent to a mailing list is delayed, it is required to add the domain from the mailing list address in the white list.

• Disable Greylisting spam protection on the whole server by unchecking Switch on server-wide greylisting spam protection in from Plesk > Tools & Settings > Spam Filter Settings

  Warning: This change reduces the protection from spam.