Port 465 is for smtps - SSL encryption is started automatically before any SMTP level communication.
Port 587 is for msa - it is almost like standard SMTP port. SSL encryption may be started by STARTTLS command at SMTP level if server supports it. Standard SMTP port accepts email from other mail servers without authentication. MSA should accept email after authentication (SMTP AUTH). It helps to stop outgoing spam when netmasters of DUL ranges can block outgoing connections to SMTP port.
Port 587 is for msa - it is almost like standard SMTP port. SSL encryption may be started by STARTTLS command at SMTP level if server supports it. Standard SMTP port accepts email from other mail servers without authentication. MSA should accept email after authentication (SMTP AUTH). It helps to stop outgoing spam when netmasters of DUL ranges can block outgoing connections to SMTP port.
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