Now and then, I've made half an attempt to make my Gnus archive messages in an IMAP box, but so far, I've failed with various spectacular error messages.

Today I managed to get just the right keywords in my google query, and ended up at this blog post, which gave the answer:

(setq gnus-message-archive-group "nnimap+<select-method-name>:INBOX.Sent")

It seems to work, even after a restart of emacs. The real test is to see if it works tomorrow too! :-)

The funny part is that the blog post actually refers to my page on Gnus+nnimap+Courier IMAP+SSL. It's always funny when you're referred back to your own pages when looking for information about something you need help with :-).