While looking at the summary of your message press A to attach a message in one of your spools. You'll be prompted for the mailbox. Then, tag (t) the messages you want attached and quit (q).

If you want to flush your inbox and move the messages you've read into the =read box without leaving mutt, just revisit your INBOX again by pressing c!.

Starting with Mutt 1.5.7, you can have Mutt cache IMAP headers to improve IMAP performance considerably. Set the header_cache option in your ~/.muttrc:

set header_cache=/home/bob/.cache_dir

Press T and enter a pattern like:

~s foo

or:

~c bogus@example.com

After you've tagged the messages you want to operate on, press ; followed by an action (e.g. s some_folder).