A developer is configuring an internal application that needs to relay messages through the company's on-premises mail gateway. The gateway accepts unencrypted SMTP connections that will later be processed by a spam filter and then routed to Microsoft 365. The firewall currently blocks all outbound traffic from the application server. Which single TCP port must be allowed so the messages can leave the server?
Unencrypted Simple Mail Transfer Protocol (SMTP) sessions use TCP port 25. Enabling that port allows the application to open an SMTP session with the mail gateway and submit messages for relay.
TCP 110 and TCP 143 are used by POP3 and IMAP, protocols that clients use to retrieve mail, not to send it.
TCP 443 is used for HTTPS, which secures web traffic and is unrelated to standard SMTP relay. Allowing any of those ports would not let the application transmit outgoing mail through the SMTP gateway.
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