In a few days our Mail Hoster (IONOS Germany) intends to activate further restriction for mail submissions.
For all submitted mails the mail sender domain has to match the mail domain of the user who submits the mail, where "user" means the user name (email address) for authentication the mail submission attempt.
All of our 20 users submit their mails with personal sender addresses to MercuryS, and Mercury C is submitting all of them to the mail provider via the one and only MercuryC user credentials. But all personal email addresses have the same mail domain like the "submitting user" defined in MercuryC. Should work.
So far so good. But in case I've set some Mercury filters where certain incoming external mails will be directly forwarded to the internet again (bouncing), the sender address will not be replaced by a local mail address. And this will lead to a refuse of that mail submission.