Receiving an email that refuses to follow Gnus’ client-side filter rules?

(setq nnimap-split-fancy
        ("subject" "media alert" "INBOX.pressreleases")
        (many other rules here)
        ;; Default mailbox

If running B q on the message shows you the filter rules do not apply, then perhaps the Subject header is encoded.

Check the raw message (c-u g), and if it looks something like:

Subject: =?utf-8?B?GJIxnJEgYWxlcnQ6VE5hYWRsb29mVUAjaTSeMJkyoiB0dXNzZW4gd2VynlOy?=

then this will help:

; Decode headers before splitting on them:
 (setq nnmail-mail-splitting-decodes t)

Thanks to Adam Sj√łgren on usenet’s gnu.emacs.gnus