Notmuch: (un)tag outgoing mail | Whisper of the Heartman

I'm here to save past me from spending hours searching the internet for information on how to make notmuch (the Emacs client, specifically) not apply default tags to outgoing mail.

The problem

So you've managed to set up OfflineIMAP to fetch mail from your server and you've set up msmtp to be able to send mail. It's taken hours (many more than you care to admit), but you've finally got a functioning email client directly embedded in Emacs! This is a good day! You celebrate by firing off an email to another email account you've got set up and ... what's this? You've got an unread email from yourself in your inbox?

On closer inspection, you realize it's the one you just sent off. But why is it in your inbox? And why is it tagged 'unread'?

Notmuch is a tag-based email system. Rather than folders, everything is done
through tagging, filtering, and searching. As such, your 'inbox' is just all
mail that's tagged 'inbox'.

You check your .notmuch-config and find that you've told notmuch to automatically tag new mail (pulled from the server) with 'inbox' and 'unread'. This makes sense, you think to yourself. After all, you'd like to get new mail into your inbox and you'd like it to be marked as unread. But why would notmuch apply the same tags to outgoing mail?

The solution

After spending hours trying to make notmuch's post-insert hook work (and still not quite understanding that one), you decide to check out where sent mail is stored, thinking you could use the location to filter the email that gets processed by the [new] section of your config. You remember reading that it relates to the 'Fcc' field, so you check out the description for the variable notmuch-fcc-dirs and find this nugget:

[...] the header should be of the form "folder +tag1 -tag2" where folder is the folder (relative to the notmuch mailstore) to store the message in, and tag1 and tag2 are tag changes to apply to the stored message.

Can it really be that simple? Is that really it?! All these hours for that? You set it to sent -unread -inbox and fire off an email.

...

Yup. That's it. Your message template now looks like this and behaves the way you want it to, both removing 'unread' and 'inbox' tags and adding a 'sent' tag so you can easily see all your sent mail:

From: Your Name &lt;yourname@provider.tld&gt;
To:
Subject:
Fcc: sent +sent -unread -inbox
--text follows this line--