mails get flagged in OutLook

[Nuno Silva]

On 2026-02-09, Una Smith wrote:

>>     On Monday, February 9, 2026 at 09:53:53 AM MST, Matthias Apitz <guru at unixarea.de> 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