Email gets easier to discuss when each system has a role instead of everything being called “the mail server.”
Common architectural roles include:
- MUA — Mail User Agent used by a person.
- MSA — Message Submission Agent accepting submitted mail.
- MTA — Mail Transfer Agent moving messages toward destinations.
- MDA — Mail Delivery Agent performing final delivery into a message store.
Real products can perform several roles at once.
This vocabulary is especially useful when explaining where authentication, routing or message modification occurs.