[dima55]

A case for mu: it is NOT tag-based, which is a feature: it can work with normal IMAP, and thus other email clients that aren't mu can look at that mailbox, and have all the data. Notmuch's tags do NOT live in IMAP (as far as I know), so if your workflow relies on them, you get annoyed if you ever look at your mailbox in any other way.

With mu, you just do the normal thing: instead of tagging a message as "x", you put it in folder "x". Clearly this breaks down if you want to apply more than one tag to a message, but this hasn't been an issue for me in the 20+ years I've been using email.

Before I read this, I had no idea that mu even did any tag-based anything.

[nextos]

That's not true. You can still have Notmuch to play well with other mail clients. You simply need to work out a good mapping between tags and folders, which is trivial.

IMAP only supports a few tags: read, unread... But not custom ones.

The only possible scenario where Mu is better than Notmuch is where you have many many custom tags and you want them to be perfectly synced across Emacs instances in different computers. Since Mu stores them in messages, this is very easy. With Notmuch, you'll need to separately sync the database. E.g. by rsyncing it.

But note that other clients won't understand these tags.