Messages are not delivered: lost connection while performing the HELO handshake

Symptoms

• Unable to send mail messages from example.com to particular domain with the following error in /var/log/maillog :

  Dec 1 04:09:54 hostname postfix/smtp[28583]: 890201800C1C: to=<[email protected]_domain.com>, relay=mail.some_domain.com[203.0.113.100]:25, delay=244006, delays=244006/0.01/0.07/0, dsn=4.4.2, status=deferred (lost connection with mail.some_domain.com[203.0.113.100] while performing the HELO handshake)

• Messages to other domains are delivered successfully.

• Checking RDNS for mail server IP shows that several domains are associated with IP:

  # dig +short mail.example.com
  203.0.113.2

  # dig +short -x 203.0.113.2 | wc -l
  112

• Server hostname resolves to the same IP:

  # dig +short server_hostname
  203.0.113.2

• PTR record is added to DNS Template under Tools & Settings > DNS Template ;
  

Cause

Misconfiguration of Reverse DNS, so the sent message does not pass the validation procedure for incoming SMTP connections on the recipient side.

Resolution

The best practice is when reverse DNS request returns the server’s hostname, which by default is the same as mail server hostname.

To configure PTR correctly follow the steps:

1. Delete all PTR settings from DNS Template and apply the changes;
2. Create a domain in Plesk with name “server_hostname”;
3. Create PTR record for the domain under Domains > server_hostname > DNS settings, which points to 203.0.113.2.
4. Check Send from the specified IP addresses (203.0.113.2) option under Tools & Settings > Mail Server Settings :
   

Additional Information

SMTP greeting contains the hostname only: lost connection while performing the HELO handshake

How to add PTR record to Cloud Server with installed Plesk?