I still use mutt on a daily basis.
Being able to select emails using regular expressions is super useful.
IMAPFilter is also simple and powerful to quickly sort email.
What do you use to fetch email? mbsync keeps messing up the UID of emails with gmail.
Yes! I use mutt for searching, tagging, thread operations (splitting,appending etc.), bouncing, attaching other mails, deleting attachments, openpgp (personal), smime (at work), and it works with IMAP out of the box.
I am also able to integrate (a locally hosted) LanguageTool to check the grammar in the editor.
The only issue is to write emails with embedded images. But personally I don't like such emails for occupying the space of the mailbox.