On 2026-02-09, Una Smith wrote: >> On Monday, February 9, 2026 at 09:53:53 AM MST, Matthias Apitz >> <[email protected]> wrote: >> >> I'm adding to all my mails sent with mutt (like this one) two X-header >> lines: >> >> X-Operating-System: FreeBSD 14.0-CURRENT r1400094 (amd64) >> X-message-flag: Mails in HTML will not be read! Please, only plain text. >> >> I got informed by an reader of a mailing-list that my mails get flagged >> in his OutLook mailbox with a small red flag. Technically this is added >> by a Microsoft SPAM protection which adds into my mail a header among other >> stuff a line like this: >> >> ``` >> X-Microsoft-Antispam-Mailbox-Delivery: >> >> ucf:0;jmr:0;auth:0;dest:I;ENG:(910005)(944506478)(944626604)(4710137)(4713098)(499 >> 9163)(920097)(930201)(20251009147)(140003); >> ``` >> >> I did tests with an OutLook destination I've access to and the "culprit" >> seems to be only this X-header: >> >> X-message-flag: Mails in HTML will not be read! Please, only plain text. >> >> Any comments on what could he do or what could I do? > > Your advisory "Mails in HTML will not be read! Please, only plain > text." needs to be in the body of your emails, perhaps in your > signature, not in your headers. Very few recipients ever read their > headers; many don't even know headers exist.
This seems to be displayed by at least Microsoft MUAs, so that's probably why it's being used. I don't speak German, but this has a screenshot: https://www.msxfaq.de/exchange/clients/xmessageflag.htm -- Nuno Silva
