Simple Mail Transfer Protocol Standard Port 587. Msa port 587 ports 465 and 587 are intended for email client to email server communication sending email. Ultimately starttls was the protocol chosen.
When users submit an email to be routed by a proper mail server this is the one that will provide best results. 587 is considered to be a standard one. Simple mail transfer protocol.
The confusion around port 465 and port 587 stems back to 1997 when a standard for encrypted transit was being discussed.
The email client also used port 25 smtp to submit messages using the smtp protocol. It is being used as a default mail submission port. Whereas port 25 is the recommended port number for smtp communications between mail servers ie for relaying messages port 587 is the one recommended for message submissions by mail clients to mail servers. The email client also used port 25 smtp to submit messages using the smtp protocol.