The "anti-spoofing" policies within Mimecast are configured on a customer by customer basis. These policies are a fairly blunt tool, and are set by default on new Mimecast accounts. They can apply to the "From:" header, the sender envelope address or both.
The member should be able to take it up with their IT folks who can adjust the Mimecast policies accordingly. Such policies are known to cause trouble in these sorts of scenarios. Hope this helps... Best wishes, Matthew (NOT from Mimecast, but I have clients who use it) ------ >From: Julian Bradfield via mailop <mailop@mailop.org> >To: mailop@mailop.org >Cc: >Date: Tue, 5 Mar 2024 21:01:57 +0000 (GMT) >Subject: [mailop] mimecast "antispoofing" >I just had a bounce, when a member of the list posted, from what is >presumably a mimecast served company, to one of my club lists. >The member's own copy of the list posting was bounced by his provider >with > > 550 Rejected by header based Anti-Spoofing policy: > [member's address redacted] - > https://community.mimecast.com/docs/DOC-1369#550 > [_gg8dmJZP_u-QdQxhMDwLg.uk114] > >I suppose that this means they don't accept a message "From:" an >address they serve - but the message was DKIM-signed by the company my >member sent from, so they should know it's not a spoof! (And my list >is DKIM-clean.) > >What should they be doing according to "best practice"? > >Julian. > >_______________________________________________ >mailop mailing list >mailop@mailop.org >https://list.mailop.org/listinfo/mailop _______________________________________________ mailop mailing list mailop@mailop.org https://list.mailop.org/listinfo/mailop
