Emacs' Gnus search and marking messages by email header

“This is a note, more to myself, on Emacs' Gnus (certainly among the best email clients ever).

Gnus offers multiple ways to search and find messages in mailboxes or on mailservers. Here is a handy one, finding and marking messages by mail header:

in a mail box, hit ‘&’, select header, fill in the desired value, select type of mark (E for expiry, for instance).

more here.