I have a vps with Centos 6.5 and postfix installed. When I want to connect to a smtp server like google or mail gun, I get the error 502 command not implemented. Can anybody help me please?

This is the full story:

[root@mail ~]# telnet smtp.gmail.com 25
Connected to smtp.gmail.com.
Escape character is '^]'.
220 smtp.passthru
250 smtp.passthru
502 Command not implemented

I think the problem is "smtp.passthru" ... What is that?!

  • 1
    Someone is intercepting your traffic. Oct 28, 2014 at 23:58

1 Answer 1


This SMTP exchange is generated by Copfilter, an addon to the IPCop firewall.

Its presence here indicates that your VPS service provider is intercepting outbound SMTP traffic in order to filter it.

Typically Copfilter is used to run anti-virus and anti-spam checks on email, though it could be used for other purposes.

In other words, instead of reaching Gmail's servers, you have connected to the firewall, and the firewall will decide whether to pass your email along. It could also do other things such as keep a copy of the email you send.

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