Final answer:
The standard TCP port for SMTP is 25, but port 587 or 465 may be used for secure email transmission using TLS or SSL respectively.
Step-by-step explanation:
When configuring a computer's email for the Simple Mail Transfer Protocol (SMTP), the standard Transmission Control Protocol (TCP) port number used is 25. This is the default SMTP port for sending emails. However, due to security reasons, many email providers now prefer using port 587, which requires a secure connection using TLS, or port 465 for SMTPS (SMTP over SSL). It's essential to choose the correct port number in email client settings to ensure that outgoing emails are properly sent and security is maintained.