...or at least arbitrary text somewhere the header.
This was discussed in [SOLVED] Searching message headers? and hinted at in Bug 7384 - IMAP search on non-indexed header returns error but I'm not entirely clear on the outcome.
That is: you can create filters which act on arbitrary header fields, which is nice because among other things, you can Filter on X-Spam-Level if you think the spam system isn't stringent enough for your needs. And, apparently as of ZCS 5.0.5, IMAP clients can request server-side searches on arbitrary headers.
But is there a way to do such a search in the web client? Among other uses, if I was thinking about creating a Filter that does something drastic, I'd like to know beforehand whether any of my existing mail would have been affected. (See Bug 37253 - RFE: add ability to preview filter/optionally apply to existing mail.)
Right now it seems the only way I can find something in the header is if it's in a field that's already offered via Advanced Search. (Yet there's a Zimlet that can identify messages based on X-Mailer, hm...)