I have configured squid from source code and running as transparent proxy server everything is working fine browsing FTP caching. Email client from Thunderbird are able to connect SMTP server but outlook clients are not able to communicate with SMTP server on port 25 therefore i am using 587 for emailing.

I have another Squid server by outlook user are able to send emails using port 25. I am not understanding what is the issues.



  • Please add both outlook version, and they are up to date on each computer ? – yagmoth555 Sep 9 '16 at 12:45

SMTP and port 25 are not HTTP. Squid is an HTTP proxy and has no ability to proxy port 25 traffic safely. Just because both protocols use MiME format for their headers does NOT make them equivalent or even compatible.

If you try to send SMTP port 25 traffic to a transparent (interception?) HTTP proxy the SMTP protocol will be treated as non-compliant HTTP messages and become all mangled up by the MiME header changes that HTTP proxies are required to do to make the messages compliant with HTTP.

This and the corresponding security problems such changes cause is why port 25 (and others) is excluded by the Safe_ports ACL of squid.conf.

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