Seems like the number 3 item you would want to nail down in a mail-client. But I'm sure it is just me that haven't looked into it yet. Unfortunately I just can't find time for it for the time being.
I'd like a mail client as capable as mutt, but that also works whether I'm online or not, handles mail sync automatically in the background while I start processing those mails, lets me browse and search and draft responses offline, and has a built-in idea of "has that mail been sent yet?" rather than just saying "sent" the moment a mail goes into another queue the client doesn't manage without giving any status for when it actually gets sent. And all that while also leaving mail on the server so that other mail clients such as K-9 mail on my phone (which can do all those things) can also process mail.
Sometimes I like "programs that do one thing well". But doing mail well involves taking full responsibility for mail.
FWIW aerc does have all this built-in, but also offers the possibility to just hand off responsibility to external programs. As far as I'm concerned, I use the built-in email sender. Unfortunately, the built-in emails fetcher does not work well with poor internet connection or offline. That's why I use offlineimap to sync maildirs every 5 mins, and notmuch is just the icing on the cake for fast search/indexing.
