My guess is that Amazon is only looking at the SMTP MailFrom address, and you can solve the problem by choosing the option to enable SRS on forwards.
Antispam (from left menu)...
Options (section of Antispam page)...
Enable SRS when forwarding messages: [ON]
Of course, none of this would be an problem if all of the members were in his domain. Most mailing lists of this type use an external Mailing List Manager program, not the Mailing List feature of SmarterMail.
Now comes the tricky part: Lists have ongoing trust issues with the filtering software at receiving domains.
Ideally, your configuration should ensure that the list does not allow impersonation. This is tricky because:
- the posts are arriving over unauthenticated SMTP,
- enforcing SPF PASS is difficult because many organizations have incorrect or non-existent SPF policies, and
- DMARC-enforcing domains are a tiny subset of all domains that may subscribe..
Presumably, posts to the list should come from personal messages, not mass mailers, so the domain of the message From address should match the domain of the SMTP MailFrom address. (SMTP MailFrom addresses may be encoded with BATV, SRS, or equivalent, so it will be easier to prove same-domain than to prove same-user. )
You can turn on SPF and DMARC enforcement, but these are system level settings. For the events that add weights, different domains could have different weights, but nothing is going to be tailored to the mailing list specifically.
Will the mailing list (or your spam filtering) add content to the mailing list messages? Doing so will break DKIM signatures which can cause messages to be blocked by some recipients' mail filters. Of course, if the list causes messages to be blocked for any reason, your server's reputation can be affected, which could have ripple effects for other clients. Ideally, the outbound messages should have a dedicated outbound gateway so that your other clients will not suffer if this occurs.
In short, I would do everything in my power to isolate the list traffic from everything else.