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How should I check if my ISP blocks port 25?
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To test if port 25 is open or closed, click the "Start" button and type "ipconfig." Type "telnet mail.domain.com 25," substituting your Internet Service provider's domain name for "domain" and hit the Enter key. If port 25 is closed, you will see a connection error.
cmd> telnet <some well known email provider IP> 25
to determine which exactly host (subdomain) is listening port 25:
nslookup -q=MX <top-level domain>
For example:
cmd> nslookup -q=MX gmail.com
gmail.com MX preference = 50, mail exchanger = gsmtp147.google.com
gmail.com MX preference = 50, mail exchanger = gsmtp183.google.com
gmail.com MX preference = 5, mail exchanger = gmail-smtp-in.l.google.com
gmail.com MX preference = 10, mail exchanger = alt1.gmail-smtp-in.l.google.com
gmail.com MX preference = 10, mail exchanger = alt2.gmail-smtp-in.l.google.com
gsmtp147.google.com internet address = 209.85.147.27
gsmtp183.google.com internet address = 64.233.183.27
gmail-smtp-in.l.google.com internet address = 64.233.183.114
cmd> telnet gsmtp147.google.com 25
220 mx.google.com ESMTP l27si12759488waf.25
On Linux, you can 'dig', I guess.
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