From the Wikipedia articles email and email agent, I gather that there exist software packages known as message submission agents (aka mail submission agents).The Wikipedia article on message submission agents does not provide any specific examples of software implementing MSA functionality. It notes that MTAs [can] perform the function of an MSA, but also that:there are ... programs that are specially designed as MSAs without full MTA functionality.I would be grateful for examples of such software that are available for Unix or POSIX operating systems, especially examples that are packaged for one or more GNU/Linux distributions. The name of the piece of software, plus some evidence that it is indeed an MSA and available for Unix/POSIX, is all that is required. Feel free to include a small amount of additional explanation if you wish.In the event that multiple answers are posted, I will up-vote all accurate and on-topic answers, and will accept the most comprehensive of these answers as correct.Note: I am not asking for recommendations, endorsements, or opinions, so I believe this question is on-topic here.
Examples of (not recommendations for) Message Submission Agent (MSA) software
email;mail transport agent