On 2020-10-27 08:01, Silvan M. Gebhardt wrote:
I assume this swiss provider has never allowed any customer to leave.
Every customer who changes DNS providers becuase their marketing decides on a new website and not telling anyone and then their marketing department hijacking the DNS, and forgetting that they run some mailing via some application hosted with you and not caring about spf records becuase the webdesigner doesn't know it, will get you on the blacklist.
Mail server admin can do a SPF check (or have a list of allowed source email domains) before outbound and reject forwarding these emails.
Of course the marketing/sales department will just "make it work" and "that is your problem"....
Still, technically, something one can stop from wrongly forwarding messages that the host is not supposed to be sending.
Given that one really need properly configured SPF/DKIM/DMARC to have your mail not arrive in Spam folder on the big providers.... it does not make sense to send mail that one cannot send anyway.
Greets, Jeroen