Kyriakos Terzopoulos writes:
Some research showed me that I needed a rDNS entry in my server. I'm on a Hetzner cloud VM so I used the Hetzner console to change the rDNS entry from:
static.XXX.XXX.XXX.XXX.clients.your-server.de
Don't do that. Aside from inducing the 127.0.0.1 problem you experience, some recipients insist on a reverse lookup that "round-trips", so that if you look up the HELO name, and then look up the IP returned, you get the HELO name back.
Oct 24 14:17:47 2023 (3759611) <169814622178.3759638.4589434779084932533@localhost> *response exception: (550, b'Helo name contains an IP address (HELO was [127.0.0.1]) and not is valid'*, 'testlist-bounces+myusername=myproject.eu@myproject.eu')
I don't understand why you get the loopback IP address in HELO. Maybe somebody else can explain.
The weird thing is that when I do a mailman-web sendtestemail this email is sent successfully.
That's not entirely surprising. When you send mail via a Mailman list a lot of processing gets done and it passess through three separate processes that are basically doing Internet mail: the incoming MTA, Mailman, and the outgoing MTA. mailman-web sendtestemail there's only one Internet-capable agent, which is the outgoing MTA.
Steve