Final answer:
SMTP uses port 25 for email transmission. It's the standard protocol for sending, receiving, and relaying emails, although alternative ports such as 587 or 465 might be used for specific purposes like message submission or secure transmission.
Step-by-step explanation:
The Simple Mail Transfer Protocol (SMTP) uses port 25 for the transmission of email messages across the Internet. SMTP is the standard protocol used by email servers to send, receive, and relay outgoing emails. While port 25 is the default, some networks may use port 587 for message submission or port 465 when using SMTP Secure (SMTPS) which encrypts the message using SSL.