Acronym for Mail User Agent. More commonly referred to as a MailClient.
IE: Your mail client, such as Mutt, PINE, Elm, Gnus or MozillaMail.
a MUA sends email via a MSA to a MTA, which passes it along to other MTA's until it reaches it's final destination, where it is passed to a MDA to deliver it to mailspool which is read by the MUA.
A MUA may access a mailspool remotely via a MAA.