SUBMISSION
A Mail Submission Agent (MSA) is a computer program or software agent that receives electronic mail messages from a Mail User Agent (MUA) and cooperates with a Mail Transfer Agent (MTA) for delivery of the mail. It uses a variant of the Simple Mail Transfer Protocol (SMTP).
Available to all SRX running 12.3X48+
Available to all MX running 20.2R1+
Available to all vSRX running 20.3R1+