Some Internet Service Providers do not allow their customers to connect to third-party SMTP (outgoing) mail servers on the default port 25. You can safely use the following ports for outgoing mail with our service:
port 587 - Both encrypted (via STARTTLS) and unencrypted connections are supported.
port 465 -  Only encrypted connections (SMTPS) are supported.
These ports are rarely blocked by ISPs, and it is strongly recommended that you use one of them instead of the default SMTP port 25.