The unofficial fallback SMTP submission port used when ISPs block 25, 465 and 587.
IANA registers 2525 as ms-v-worlds ("MS V-Worlds"), an individual's assignment that nothing uses — the "ms-" prefix is part of the service name, not an attribution to Microsoft. What actually listens here is mail submission: email service providers such as SendGrid, Mailgun and Mailjet offer 2525 as a mirror of their port 587 submission endpoint, with the same STARTTLS and SMTP AUTH requirements, because consumer ISPs and cloud providers frequently block the standard ports outbound. It is a convention, not a standard — the IETF's message submission spec (RFC 6409) defines 587, and no RFC blesses 2525.
Expose with care
Acceptable only as a real submission service that requires STARTTLS and authentication, exactly like 587. Because nothing standardises 2525, verify the server actually enforces both rather than assuming it mirrors 587's policy — and never run an open relay here.
$ openssl s_client -starttls smtp -connect TARGET:2525Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.