hhardy
Please make sure your cron has enough time to fully execute before it gets called again. I would recommend 3-5 minutes for the cron job.
As for the error, that means the webserver cannot connect to the mailserver some times. This can be intermittent issues, issues with too many connections open at a time, etc. You can reach out to your mail provider/server admin to see if they can find any issues with the connections.
As for why it takes 20 minutes, that's something you'll have to investigate further.
Cheers.