I've seen these headers as well. They were coming out of one
particular "groups" mail system, likely added as an artifact of that
process.
The super long headers can also cause some clients to just not display
the messages.
I'm pretty sure DKIM wouldn't include the non-standard threading
headers.
On 10/18/22 2:46 AM, Heiko Schlittermann via mailop wrote:
Grant,
thanks for your fast response.
Grant Taylor via mailop <mailop@mailop.org> (Di 18 Okt 2022 00:41:24 CEST):
I would (try to) configure my MTA to re-wrap the logical line to conform to
…
Modification of existing headers isn't something I would recommend.
Except for headers that aren't protected by DKIM.
I would naively assume that Exim (or any contemporary MTA) could deal with
logical (unfolded) header lines of largely any length.
Exim *can* process these overlong headers. But its default config
rejects the transport of messages containing them. (Probably the default
config even rejects such messages, I'd need to check.)
The *intention* of my question was more like "what do you do with such
messages", as I believe that I'm not the only one seeing them.
OTOH *iff* I'm the only one, than the issue might be scoped to a
specific sending system. Although the reporter told me, that they're
lots of such messages.
If it wasn't for the messages purportedly being from MS Outlook 16, I'd
wonder if this was some sort of attack.
The message itself wasn't suspicious and had a trustworthy sender. (But
I'm not sure if it was really an Outlook generated message, or some sort
of script impersonating as Outlook.)
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