What is the difference between SMTP port 25 (relay) and port 587 (submission), and why should mail clients use 587 instead of 25?

• A.Port 25 uses TLS encryption; port 587 uses plain text. Mail clients should use 25 for better security
• B.Port 25 is the SMTP relay port used for server-to-server email delivery (MTA to MTA); ISPs commonly block outbound port 25 from residential/business connections to prevent spam from compromised machines. Port 587 is the SMTP Submission port (RFC 6409) for mail clients to submit messages to their mail server; it requires authentication (SMTP AUTH) and uses STARTTLS for encryption. Mail clients should use 587 to avoid ISP blocking and to authenticate with their mail server
• C.Port 587 is for receiving email from external servers; port 25 is for sending email from clients. The ports are equivalent in functionality
• D.Port 25 and port 587 are identical; the choice between them depends only on the mail server software being used