I've been using the popular AVM DSL routers (FRITZ.Box) for many years now. They're affordable and mostly do a pretty good job. However, there's this one thing, that has been bothering me forever ... the FritzBox's DHCP server sets the domain of all hosts to 'fritz.box' ... and apparently there's nothing you can do about it (that's what the internet keeps telling me at least). While this escalated from mildly annoying when using the shell, to rather annoying when SSH keys seem to be coming from 'foo.fritz.box' to plain unacceptable when your mail headers look like this:
Received: from foo.fritz.box (pxxxxxxxx.dip0.t-ipconnect.de [xx.xxx.xx.xxx]) by mail.example.com (OpenSMTPD)
While it's not a solution to the underlying problem, masking the source helps to not propagate your setup to every mail server out there:
listen on egress tls pki mail.example.com auth-optional mask-source listen on egress port submission tls-require pki mail.example.com auth <passwd> mask-source
The title is somewhat exaggerating, this is for sure not a fix but just a band-aid. The next step is to replace the DHCP on the router with something sane and configurable.