The RFC documents related to mail use a common set of terms that have similar abbreviations to describe different agents, actors, or services within Internet mail services.
The Mail Submission Agent is the LMTP service (similar to SMTP) that accepts authenticated new mail messages from end users' MUA. It operates on port 587 and is intended to be the sole entry point for new mail into the mail system. Once new mail is accepted by the MSA, it is passed to the MTA for routing towards its final destination.
The Mail Transfer Agent is the SMTP service that filters and accepts mail on port 25 from outbound mail services and routes it towards a destination domain's MX for delivery to a mail store and ultimately the end-user mailboxes. Historically, before the advent of junk mail (spam), SMTP port 25 performed the functions of both MSA and MTA, but to help reduce abuse they were split into two distinct services for better management.
The Mail User Agent is the end user client software used to read and send mail, such as Mozilla Thunderbird or Microsoft Outlook. It can also be web based mail services, such as Gmail, Outlook, or Yahoo.