We see these kind of issues within hosted environments as well.
This however is quite a rare issue and in my opinion mostly is because of temporary the connection to take unexpectly long.
If this doesn’t cause mails not being sent out, I personally would say it’s okay.
Usually this appears after no longer than 5 minutes.
Zammad retries sending mails up to 4 times.
You may also want to check the mail servers logfile if possible.