Best practice - limiting SMTP traffic to email server

Whenever I have utilized a cloud-based spam filtering service, I have always adjusted the SMTP rule on the firewall to only accept traffic from said service. However, I have recently seen docs/articles directing to adjust your email server to do this task instead (i.e. locking down the receive connector on the Exchange server).

My thinking to this would be that the less work the server has to do the better, and it seems a job more suited for the firewall anyway.

Am I missing something? What would be the advantage(s) to having the email server handling this task, over the fw?


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Simon Butler (Sembee)ConsultantCommented:
I think the reason that a lot of documents say to get Exchange to do it, is simply because a lot of sites are using "home" type routers supplied by ISPs which are unable to restrict the traffic at the firewall level. Many sites will continue to use those routers because if they use something else ISP support stops if they have an internet connection. They keep the router to keep support.

These are the same sites that are unable to block port 25 traffic outbound for anything but Exchange and then get blacklisted.

If you are able to do the restricting on the firewall, then I would do it there. Also block all other SMTP traffic on the firewall as well.


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tnormanAuthor Commented:
Thanks for the time on this one, Simon.
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