On 2 Apr 2019, at 13:07, Ken Johnson wrote:
So one of the on-site engineers had the notion to change MUAs from
Outlook
2016 to Claws Mail. Now his email to Large Organization is being
delivered.
If you had this problem, where would you start looking?
Are you DKIM-signing mail? How? Are you using Sendmail?
I have seen Outlook routinely submit mail with To and/or CC headers with
display name parts that offend Sendmail due to unquoted "special"
characters. If you sign that with a milter, you sign the submitted
format. Then Sendmail takes the signed message and enquotes the risky
display names, breaking the signature. Many sites deem DKIM checking
synchronously at SMTP end-of-data time to be a burden, so they accept
and queue for verification. In some cases (especially with senders that
publish p=reject or p=quarantine DMARC policies) they will not deliver
messages with broken signatures but they also can't responsibly do
asynchronous bounces, since they already suspect the message is a
forgery.
Generally speaking, a MUA change that impacts deliverability of 1-to-1
mail is *likely* to be the result of some sort of bad formatting by the
MUA that yields poor deliverability, whether it's a problem that passes
through to the end site before being shunned or one that gets 'fixed' in
transit only to break something else (like DKIM) as a side-effect. When
Outlook is involved, it is the chief suspect.
--
Bill Cole
[email protected] or [email protected]
(AKA @grumpybozo and many *@billmail.scconsult.com addresses)
Available For Hire: https://linkedin.com/in/billcole
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