If 25 is being blocked you may have to use 587. If you on a dynamic IP. Well my
guess is your not setting behind a router/firewall on your SOHO network. So the
fast way around a dynamic IP is to use a DDNS like no-ip.com or dyndns.com . I
know no-ip has a client for linux. (use it on guilds mumble server). So it
should work for postfix to. Got to love free sub-domains for redirect of
servers.