This is mildly interesting: I have a Postfix server configured to offer SASL on port 587. Dovecot is providing the SASL support. Microsoft Outlook 2016 for Mac fails to send messages out via my server, reporting that it doesn't support any of the available authentication methods. Meanwhile, Microsoft Outlook for Windows can send messages via the server without any difficulties. I've played with configuration settings, but with no luck. If I try to enable logging on Outlook for Mac, various operations (including, crucially, creating a new message) become unavailable, and it never seems to finish reading the inbox. Without logging enabled, it's able to read IMAP folders fine. This doesn't really matter - I have plenty of other ways of sending mail, even when using the Mac, but if anyone here has clues about the mystery, I would be interested. It's worth noting that Outlook 2016 for Mac works reasonably well with VoiceOver. Unfortunately, it doesn't implement equivalents to many of the keyboard shortcuts (especially for commands in the ribbon) found in the Windows version. Thus, for example, changing to a different folder is a slow process - I haven't found a faster method than scrolling through the list with cursor keys. Nevertheless, someone at Microsoft has evidently been paying attention to its accessibility, and that's a positive development.