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For security reasons.
SMTP (Simple Mail Transfer Protocol) is used for terminal device (computer, tablet, smartphone) and mail server connection, and also for communication between mail servers. Port 25 was previously used for both communication types, i.e. to send outgoing e-mails from e-mail programs (Outlook, Windows Live Mail, Thunderbird, etc.) to mail servers and then send to them from this mail server to the recipients' mail servers.
However, in the past port 25 was frequently misused for spam mail. In 1998 this resulted in a new standard, which now runs terminal device communication to mail servers via port 587 instead of port 25.
Swisscom takes the battle against spam seriously. For security reasons, we have decided to filter outgoing e-mails to e-mail servers via port 25 in our network and only allow them under specific circumstances.