This is a heads up that some people might find useful.
I was playing with Fedora 29 the other day. The default mail client for that distro is Evolution, and while it was able to set up the IMAP server automatically, I found that when I was sending messages the outbound mail was generating errors caused by the greylist module.
On investigating the outbound settings, it turned out that Evolution automatically set itself up to use port 25 for outgoing mail. As port 25 is normally used for inbound this was triggering the postgrey filter.
The solution was to go into the account settings and change the outbound port to port 587 with SMTP authentication enabled.
This does not happen with all mail clients, but it is something that MIAB admins should be aware of.