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SMTP port (TCP/25) to legitimate (MX) server intercepted by Swisscom

1 of 6

Hello Swisscom,


While performing functional tests on my company's newly-migrated Mail-eXchange/MX server, I found out that Swisscom (inOne Home Internet) intercepts all SMTP (port TCP/25) connections:


$ telnet <whichever-IP> 25

Trying <whichever-IP>...
Connected to <whichever-IP>.
Escape character is '^]'.
220 Swisscom AG ESMTP server ready

[definitely not the legitimate SMTP server expected at <whichever-IP>]


And seamlessly relays e-mails presumably sent to that SMTP server via its own server (I can send you proof of this by private message if you wish); quite close to what one could call illegetimate man-in-the-middle...


In those days of SPAM, I can understand your willingness to control (potentially illegitimate) SMTP traffic on port TCP/25 (normally reserved for Mail-Transfer-Agents/MTA communications) but I would expect such connections to be rejected with a SMTP 5.x.x error code (and a human-friendly message, eventually displayed to the user by her Mail-User-Agent/MUA) rather than being silently relayed via some unsuspected (and unauthorized) third party.


Can you elaborate on this situation ?


Thanks in advance and best regards,


Cédric Dufour


2 of 6

 Same here...

As said in the last post by  @cdufour this is a man in the middle attack at my connection!

Even worse -  instead of blocking the connection you send the mails from your own Mailserver (as shown in the email-header) which is definitly a Privacy issue...



Received: from ( [])


Super User
3 of 6

to be honest I really don't understand what you guys mean.

Why are you using unsecure smtp? 

What's your exact use case? 

Just want to understand this situation a bit 🙂

4 of 6

What about:

Running one's own MX (Mail Exchange) server - for one's own domain - which, when it acts as Mail Transfer Agent (MTA) to transfer messages to recipient MX servers (e.g., MUST use the "unsecured" SMTP TCP/25 port, as per RFC 821 (Appendix A), further clarified (distinguished from Mail Submission Agent (MSA)) in RFC 2476 (Chapter 3.1), and explained on 

In the context of MTA, SMTP TCP/25 is thus *the* port to use and in not "unsecure" for modern-age MX/MTA servers, which will STARTTLS to enable end-to-end encryption of the transferred message(s), as per RFC 3207.

Swisscom acting as man-in-the-middle thus: 1. forfeits the purpose of STARTTLS and allows an unauthorited party (Swisscom) to see the "cleartext" content of the transmission (which it is not the intended recipient); 2. will break the Sender Policy Framework (SPF, RFC 4408) verification performed by the receiving MX/MTA server (since the relaying server - that of Swisscom - is *not* the authorized one for the sending domain).

Again, if Swisscom feels that SMTP TCP/25 is too much a SPAM (origin) threat for its *residential* customers (which can reasonably not be expected to run a MX/MTA server), it should either: A. reject all sending attempts with a STMP 5yz code (RFC 821, Appendix E); or B. Block the TCP/25 port altogether.

Of course, one wouldn't dare imagine Swisscom performing such man-in-the-middle for its *business* customers, whom one can very reasonably expect to run a MX/MTA server for their own IT purposes. (just curious, however...)

Super User
5 of 6
Ok must pass on this topic.
Nothing I ever thought about. Perhaps somebody else has a hint for your.
New Member
6 of 6

Not cool, I just lost half day trying to figure it out why my message got rejected by swisscom when I was using postfix from my ubuntu and google.