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 587 is for receiving email from external servers; port 25 is for sending email from clients. The ports are equivalent in functionality; TFTP performs block-level resume by re-sending the entire file from block 1. RTP media streams require sequential even-odd TCP port pairs below 1024. DNS zone transfers ride UDP 53 because TCP cannot carry records larger than 512 bytes. Traceroute on Windows sends UDP probes while Linux tracert sends ICMP, the reverse of their real defaults
• B.Port 25 uses TLS encryption; port 587 uses plain text. Mail clients should use 25 for better security
• C.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