As I already wrote in a post in May 2010, Switzerland's biggest Internet Provider (Swisscom) doesn't allow to use port 25 to send e-mail anymore (except when using Swisscom's SMTP servers).
The interception can be seen with a simple telnet to the web server of my website (area-1.ch):
telnet area-1.ch 25
220 zhhdzmsp-nwas18.bluewin.ch ESMTP Service (Swisscom Schweiz AG) ready
250 zhhdzmsp-nwas18 says HELO to 18.104.22.168:49765
Instead of area-1.ch, the response comes from zhhdzmsp-nwas18.bluewin.ch. Clearly an interception (or port forwarding on the provider site).
For all independent web hosters this is such a pain, and even more for their customers as they must manually switch the SMTP port in their mail clients.
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