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The SMTP that we're talking about uses port 25, not port 587 (or 465). But yes, an ISP could block the STARTTLS negotiation. Port 587 is submission - an end-us
by Repulsion9513 2y ago
The SMTP that we're talking about uses port 25, not port 587 (or 465). But yes, an ISP could block the STARTTLS negotiation.
Port 587 is submission - an end-user's client talking to their email provider's SMTP server.
Port 25 is smtp - your email provider's SMTP server talking to the recipient's email provider's SMTP server.
Port 443 is https - not TLS. TLS can operate over any port you like. The port is determined by the service being used, not by the fact that it's TLS.