Hi,
thanks for the reply.
1) Well yes, but no as well. It's related to DMARC/DKIM. So if user@external.com sends an e-mail or meeting to user@rayn .com and user@rayn .com clicks forward and sends it to user@Stuff .com what happens for e-mail is that the from from is rewritten to user@rayn .com and the outgoing server should obviously adhere to DKIM/DMARC. For meeting however the from is not rewritten and remains on user@external.com. If DMARC for external.com then has a reject policy and forwarded-to-receiver (like gmail.com) checks DMARC the message will be refused as local.com can't DKIM sign for external.com nor will it be listed in SPF for external.com.
So yes, it's specific as in not all domains have DMARC reject policy and not all receivers check it, but if both are true it will happen for all of them.
2) Yes you will receive NDR if the receiver sends reject messages for DMARC. This is where it gets really annoying, gmail for example will nicely send a reject message for this. A lot of mail servers however accept the message, but then quarantaine or discard the message silently. This is very receiver configuration specific of course.
The NDR gmail returns:
user@Stuff .com
[IPv6]
Remote Server returned '554 5.0.0 <[IPv6] #5.0.0 smtp; 550-5.7.26 Unauthenticated email from original-sender.com is not accepted due to 550-5.7.26 domain's DMARC policy. Please contact the administrator of 550-5.7.26 original-sender.com domain if this was a legitimate mail. Please visit 550-5.7.26 https://support.google.com/mail/answer/2451690 to learn about the 550 5.7.26 DMARC initiative. SOME-ID.269 - gsmtp>'
Note this also tells you to talk to the administrator of original-sender.com, but they can't do much about a user on another domain forwarding the meeting (works fine for e-mail thus as from is correctly rewritten).