2025-09-21 22:45:00
I'm setting up Mailcow for self-hosted email. Lots of moving parts and one of the stranger things was this bounce message when emailing to my MS365 account.
<tess@${MyDomain}>: host ${MyDomain}.e-v1.mx.microsoft[52.101.68.18] said:
550 5.7.1 Service unavailable, Client host [178.zzz.yyy.xxx] blocked using
Spamhaus.
That IP address was not something I'd outright recognized as my own. I know exactly which IP address is tied to my static fiber connection, so I was confused about this one! I also couldn't do a reverse DNS lookup, the IP did not link back to a name.
It was only while doing some Rubber Ducky Debugging on the Angry Nerds Discords that I realized what was happening: that IP address belongs to my fallback 4G GSM router!
More than two years ago I'd setup a Teltonika RUT241 as the secondary WAN interface for my Unifi router. It works like a charm and load balancing makes it possible to have faster downloads than my single fiber allows.
But load balancing also meant that some emails were being sent over the GSM WAN. And that's not cool. :D
It was easily fixed by adding a policy-based route, ensuring that all hosts in the DMZ would only communicate with the Internet via the fiber on WAN1.
kilala.nl tags: sysadmin,
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All content, with exception of "borrowed" blogpost images, or unless otherwise indicated, is copyright of Tess Sluijter. The character Kilala the cat-demon is copyright of Rumiko Takahashi and used here without permission.