the socket connection to the remote host failed.

[kiamori]

Ran into an issue today with a client having issues sending to a specific mail server, and it took me a bit to figure it out so I thought I would post it here in case someone else has the same issue.

Error:

The socket connection to the remote host failed.

NDR Error:

Reason: Mail Server for recipientdomain.tld could not accept your email and the message as been retried too many times.

The issue was the remote server was taking to long to send the initial welcome message.

To solve this I went into "SMTP properties" > "Outbound" >

Under Advanced, set the "Timeout for remote mailservers" to 600.

RFC compliance states 5 minutes, so 300 seconds.
https://datatracker.ietf.org/doc/html/rfc5321#section-4.5.3.2.1

This is a temp solution and should not be set beyond 300 (5 Minutes) in my opinion. I have notified the remote email provider to solve the issue and will be returning our timeout to 300 within a few days.