I’ve been using mu/mu4e for 5 years and love it. It really provides everything I want in a mail client except perhaps for handling HTML—which certain miscreants insist on sending to me—but even that’s an Emacs shortcoming that will undoubtedly be remedied in the future. When I first started using mu4e I thought I would have to install BBDB to help me map correspondents to their email addresses.
That turned out to be unnecessary: by default, mu4e uses the mailabbrev mechanism to complete mail addresses so it just sort of worked out of the box. As far as I can tell, this works by scanning stored emails to get possible completions. The problem is that old and erroneous addresses also come up so it’s sometimes hard to know which one is correct.
I just saw this reddit post by astoff1 that recommends using ecomplete for mail address completion. This has the advantage that the mail addresses are explicitly listed in a file (~/.mailrc) so there’s no ambiguity with old or broken addresses. The downside is that you need that file of addresses.
I’m going to do a bit more research to see if I can automagically generate at least a first pass at a .mailrc file. I’ve been content to let it just work for the last 5 years so I haven’t looked into how things actually work. Now, I may need to do that so that I can implement an ecomplete solution as astoff1 suggests.