dap1--- via Mailman-users writes:

 > I am getting mail bounced from yahoo and gmail smtp servers with
 > this message:
 > 
 > 554: Message not accepted due to failed RFC compliance. Too many
 > From headers. See
 > https://senders.yahooinc.com/smtp-error-codes#rfc-compliance-failures 

Waste of time, no reference to what RFC may have been violated.  And
multiple addresses in From is conformant, although unusual.  From RFC
5322 (the only RFCs mentioned by Yahoo!'s sender requirements are 5321
and 5322);

    from            =   "From:" mailbox-list CRLF

which means what you would think it means, more than one is OK.  I'm
guessing they mean RFC 7489 defining the DMARC protocol which does
specify that a message can't satisfy "From alignment" with more than
one From address.  But that's conformant if all From addresses are
p=none domains.

 > What would cause mailman to build too many from headers when
 > distributing email? It appears the from address is repeated 5 times
 > in the from field.  I'm guessing it is something in my configuration
 > but what and where?

I agree with Mark, it's not Mailman.  I checked the code and I can
only find two cases where Mailman alters the From header, anonymous
list and Munge From.  Both involve *replacing the entire content* of
the header with the List-Post address (and a comment indicating the
original address for Munge From).

Please check the incoming messages.  I think that what *may* have
happened is that

- the message as received by your site already had several From addresses,
- Mailman condensed that to one for the purpose of the DMARC check,[1]
- that one was a DMARC "p=none" address,
- Mailman left From as received when it distributed the post, and
- Yahoo! et al rejected it.

Footnotes: 
[1]  I'll be looking at this in more detail, but as I read it now we
only check the first address of multiple addresses in From.  I think
that either we should reject such messages unless DMARC Mitigation is
None, or we should check all the addresses for p=none and reject if
any have a different policy.

-- 
GNU Mailman consultant (installation, migration, customization)
Sirius Open Source    https://www.siriusopensource.com/
Software systems consulting in Europe, North America, and Japan
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