Hello,
some while ago, this list greatly helped me out with an issue in
delivering to Mimecast, as some e-mails were claimed to have a broken
DKIM signature.
After quite some research thanks to your support, we've discovered the
problem is the missing 8BITMIME support on their end.
When the Content-Encoding was 8bit, it got bounced by Mimecast, and also
flagged as erroneus by aboutmy.email (which has 8BITMIME disabled as
well, so I could finally reproduce the problem). When I switched to
Quote-Printable, the mail passed just fine.
For Postfix, a workaround (which I didn't try yet) seems to enable
force_mime_input_conversion in newer versions.
These days, a friend of mine had a comparable issue. While I think it's
not a general Mimecast problem, but related to the recipient's specific
configuration, I want to share it here in case anyone else faces this issue.
For them, the trigger was a special character (Umlaut) in the signature.
They fixed it client-side with
mailnews.send_plaintext_flowed: false
mail.strictly_mime: true
I didn't do further research yet about the side effects of this, but
maybe it helps others affected by this.
Have a nice weekend,
Florian
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