You're right about keeping an offline copy of emails - especially with the help of an indexer like notmuch or mu. But I'm not sure why you mentioned it here. In case you think that aerc supports only IMAP, it has good integration with local maildirs and notmuch. Somebody else mentioned that maildir support felt like a second class citizen. This was true during the initial phase of the project. But the developers changed their mind some time later (there's a blog post about it somewhere) and gave it a better UX.