On 2026-02-09, Matthias Apitz wrote: > Hello, > > 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? > > Thanks > > matthias
Are you sure the red flag is related to spam/junk detection and isn't just Outlook's way of showing that a message has been "flagged", with the presence of the header being regarded as a flag too? I just tested with a Microsoft-hosted account here, and 1) I can confirm the flag text still shows up, above the message, in the current version of Outlook webmail (the blue envelope one). 2) Messages with X-Message-Flag get that red flag set. Tooltip over it says "Unflag this message", and when displaying the message, it also gets a red flag between headers and body, followed by the red text "Flagged". 3) Both test messages arrived in the Inbox, not "Junk Email". -- Nuno Silva
