Port 25 blocked by ISP
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Back when I was living in Japan I switched from Layer 1 from ADSL to FTTP with vDSL and all of a sudden I could no longer connect to port 25 on my server. Despite the fact that the underlying carrier was the same (NTT-East).
After a while I got fed up with running the old smtp-forward script and did some more research. To my surprise the submission (TCP/587) and smtps (TCP/465) were as wide open as one could hope for.
A couple of configuration steps in postfix and I was up and running without much trouble. Under /etc/postfix/master.cf ensure that the line submission and smtps are not commented.
# ========================================================================== # service type private unpriv chroot wakeup maxproc command + args # (yes) (yes) (yes) (never) (100) # ========================================================================== smtp inet n - - - - smtpd submission inet n - - - - smtpd -o smtpd_etrn_restrictions=reject -o smtpd_client_restrictions=permit_sasl_authenticated,reject smtps inet n - - - - smtpd -o smtpd_tls_wrappermode=yes -o smtpd_sasl_auth_enable=yes
If the above does not work tunneling via ssh may be an option.