Why am I receiving "dkim_remote_smtp defer (110): Connection timed out" when trying to send email to a specific recipient?

Question

Why am I receiving "dkim_remote_smtp defer (110): Connection timed out" when trying to send email to a specific recipient?

Answer

This generally occurs when a recipient has not allowed your server in their firewall. The easiest way to test this is by using the 'nc' utility to attempt a manual connection to their server on port 25:

nc -w 10 -v $recipient_ip_address_here 25
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.

This command attempts to make a manual connection to the recipient IP over port 25, if this connection times out, you will be unable to send email to the recipient until an adjustment has been made to their firewall rules.