I have created a fresh install of iRedMail, on a fresh instance of Ubuntu 16.04, following the instructions here: https://www.vultr.com/docs/how-to-set-up-a-mail-server-using-iredmail-on-ubuntu-16-04
When sending myself mail from Yahoo or Gmail I receive the messages just fine. When sending mail out to Yahoo or Gmail the messages just never arrive.
In a word -- or two -- please help!
So, I found the problem. Vultr blocks ports by default, and you have to ask them to unblock them. I made the request, and they very quickly unblocked them. Problem solved!