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  • You write "Most residential ISPs block TCP port 25 (SMTP)" <-- could you elaborate on what that means. Do you mean they won't let you make an outgoing connection to an SMTP server on port 25? Or do you mean they won't let you receive a connection on port 25? Commented Nov 28, 2015 at 2:07
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    @barlop the former — they block outgoing connections on 25 from residential links to machines other than their own mail servers (or indeed to anywhere, because they might use 587 or 465 for their own servers). However, it's somewhat of an exaggeration to say that most ISPs do it. Commented Nov 28, 2015 at 4:08
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    @hobbs - My experience (and its a fair part of my job) is different. While a lot of ISP's will block traffic leaving their network with a target of port 25 (which forces port 25 traffic through their mail servers), the same is generally not true for port 587 or 465 - and indeed this makes sense. Port 587 and 465 generally REQUIRE authentication, and blocking and are specifically MUA to MTA rather then MTA-MTA - Blocking these ports would generate HUGE backlash as many companies require it so as allow roaming, accountability and not breaking SPF. Commented Nov 28, 2015 at 19:44
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    @hobbs, I never wrote that most ISPs do this; what I wrote is that most residential ISPs do this. For instance, AT&T, Comcast, TWC, Verizon, etc. do this for their residential customers, but they do not do this for their business customers. Commented Nov 29, 2015 at 19:17