MUA

What it means

RFC 5598 puts an MUA at each end of a message's journey. The author's MUA creates the message and hands it to a Mail Submission Agent; the recipient's MUA fetches it from the mailbox, displays it, and is where replies and forwards start. The RFC counts automated senders and automatic responders, such as out-of-office notices, as MUAs too.

For senders the recipient's MUA is where presentation happens: how HTML renders, whether images load, how the sender name and preview text appear, and whether an unsubscribe button is offered. Each program does these differently, which is why one message can look right in one inbox and broken in another.