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Scenario:

Sophos Firewall <> Server 2012 DC, DNS <> Server 2012

Issue: On the internal Server 2012 I have some apps running that require to send out some mails over SMTP. At first I was not able to establish a connection at all, thinking the Win Firewall was at fault.

However, I completely disabled the Windows Firewall (just for testing purposes) allowed SMTP Packets (TLS, SSL, Plain) (587, 465, 25) on the Sophos Firewall and tried it with a ton of different SMTP-Servers. However, I can see that DNS-Resolution is working but the connection times out. On the Sophos Firewall Live Log I do not see any dropped packets anymore and at this point I am not sure what else could be the problem. If I put the Server in the DMZ everything is working fine.

Any ideas?

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You mail server is unlikely to be sending email on ports 465 (obsolete SSMTP port) or port 587 (Submission port which usually uses StartTLS). If startTLS is available on port 25, many servers will upgrade to TLS before sending email.

Many ISPs block outgoing traffic on port 25 to prevent spambots from being functional. Try configuring your mail service to use your ISPs relay server. Some ISPs offer secure service on port 465, and do not enable startTLS on port 25. If you need to authenticate the connection to the relay server, ensure you have an encrypted (TLS) connection.

For more information:

Due to a flood of spam mostly sent by spambots, most sites require senders to properly configure their servers. As well, many ISPs block outgoing email traffic from dynamic IP addresses; and may require users with static IPs to request the ability to send email. If you want to send email directly to the internet you will need to ask your ISP to configure the PTR record for your IP address appropriately.

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