Receiving SMTP traffic, possibly spam, outside of port 25

 
Best Practices Exchange 2007:
I'm limiting my inbound traffic to accept only from AppRiver.

My MX records only point to AppRiver.

So, where's this spam coming from?


Solution:

Please be aware of TCP port 587 which is used for client submission of SMTP messages to a server.

It is implemented in Exchange 2007 by default, and while it requires authentication, if an administrator were to allow anonymous they would suddenly get tons of spam
not via TCP 25 but via TCP 587.


This other SMTP submission port, port 587, is supported on almost all mail servers. In fact, according to the relevant standard, port 587 is the preferred port for client mail submission.

Additional Links and Information:

Microsoft TechNet
http://technet.microsoft.com/en-us/library/cc512685.aspx

Larry Seltzer, Use TCP Port 587 For Mail Submission
http://www.pcmag.com/article2/0,2817,1838672,00.asp

Gellens & Klensin, Standards Track

http://www.ietf.org/rfc/rfc2476.txt
 
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