Many hosting providers block outgoing traffic over port 25 to combat spam. While this can help avoid spam abuse on the network, it does cause problems for users who are sending valid emails from their servers. By default, all mail servers use port 25 when it comes to relaying messages to other servers. Blocking this port prevents your server from being able to connect and deliver messages to any remote locations. If your provider has blocked port 25, only incoming email and local mail delivery are operational.