25, 2525, 465, 587, and Other Numbers: All About SMTP Ports

SMTP is the protocol that sends email from clients to outgoing servers and relays it between mail servers, with different ports commonly used for submission and relaying. Port 25, the original SMTP port, is now primarily intended for server-to-server relaying because many providers block it for message submission due to historical spam abuse. Port 587 is the officially recognized and broadly supported default for secure email submission using STARTTLS, while port 2525 is a reliable TLS-capable fallback when 587 is unavailable, despite lacking formal standardization. Port 465 remains in use by some legacy services for SSL-based SMTP but is generally discouraged unless required, and port 1025 may serve as a last-resort submission alternative. Major providers typically use port 587, although Gmail and Yahoo also support 465 for SSL, and deliverability depends not only on port selection but also on factors such as infrastructure, email content, complaint rates, and performance monitoring.

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