Salt Fiber SMTP
August 2024
Since a few months, I've been getting more spam emails. While manageable, I have been wondering from where they have got my address. The easiest way to figure that out would be to have one unique email address per service I sign up. If I start receiving spam on one address, I can associate it with the service that has given my address to spammers. Using a catch-all, I can even make up addresses on the spot.
And because I'm sometimes a bit crazy, I decided to selfhost the email server. Many people recommend against it, but I only really need receive capabilities, so it should not be as bad. I was wrong.
Many ISPs block port 25 TCP outgoing to avoid having spammers on their network. Ingoing should not need to be blocked though, and that is all I need for receiving email.
However, my ISP salt fiber block all traffic involving port 25. While unnecessary, I'd be fine with that if they didn't pretend that it should work.
First, you can create a NAT rule for port 25 without any issues. At least a warning would be appropriate here.
Second, the firewall settings even suggest you can allow outgoing traffic on port 25:
And to make it even more confusing, as of the 10.9.2024, their own website mentions SMTP as one protocol that can be configured on their router:
Yes, that page says IPv6, but I assume it should also apply to me. I'm still an old IPv4 customer without IPv6 (in 2024!) and for the remaining protocols, what is listed has been true up to now.
TLDR
No, it is not possible to run even a receive-only mail server on salt fiber since all tcp traffic on port 25 for SMTP is blocked in both directions. This is despite multiple pieces of evidence in their products and documentation suggesting it is possible.
If you need a better ISP in Switzerland, go to init7. I don't know if you can do SMTP, but at least they know what they are doing.