Port 25 is used to send email between servers. But many internet providers block access to this port for users. If that's the case for you, don't worry. You can use an alternative port.
We have mail servers on port 25 and port 26.
If port 25 doesn't work for you, switch to port 26 and you'll be able to send email normally.
See also: /en/support/email/setup/smtp-port/