I'm using Avast Free and Outlook for years without problems. However, since new Avast version 19.1.2360, sending and receiving mail in Outlook 2016 gets stuck when scanning my Spam mail box (IMAP). This has to do with Avast, because disabling the mail shield 'solves' it. (enabling sending and receiving mail).
I sent in a bug report, but no definite solution yet.