I am with you Filippo, but it would have been a too big step for our boss. Thus the instruction was to keep the external mail provider, while all other ressources were newly engineered, new ad domain, nethserver nextcloud for caldav/carddav, neth ips, proxy, etc. etc, so I came up with this solution, to get a domain ourdomain.xy served by neth while keeping ourdomain.yz hosted by external email provider.
My task was to have an imap server on a nethserver enabling us to do additonal filtering on emails, but not to transfer the external domain to our nethserver.
Initially I thought maybe mailproxy could be used to intercept and filter by rspamd but as this module is not available on the new interface, I thought it maybe will be depricated soon, and decided for the actual setup.
I will discuss your advice, and will maybe be able to convince our CEO to serve our external domain directly by our mailserver. Maybe we could make something like primary mailserver for ext. domain: our nethserver, secondary mailserver: the external one. But this will take some time for the decision, and in the meantime we will use the time to gain further confidence in our neth environement I have setup. There is also another problem with our hosted nethserver. I realized that as I am hosting my private domain mailserver on the same hoster. They have poor ip reputation at M$ and thus from time to time mails to their domain are rejected and I regularely have to ask our hoster to delist their ips from blocklist of Microsoft. And this would be a nogo for our boss, that mails would partially not get delivered, thus he wants to keep the external mailhoster for the moment.
As the setup with connector (we use imap not pop3 for fetching with getmail) worked nicely for months, I was wondering if it could be possible that one of the latest updates which were installed automatically by subscribed nethserver could have caused this, but I understand and thank you for the hint that it also could be a problem not related to neth at all but on the external imap server. Thanks for the pointer.