Message User Agent
A Message User Agent (MTA) is an application that creates and sends emails.
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Role
An MUA accesses a remote mailbox, often via either POP or IMAP, to retrieve emails for use by or display to the intended end user.
An MUA sends emails via SMTP to a remote MTA.
MUAs generally have additional features such as:
assisting in the completion of contact information using a remote LDAP server
automated encoding of non-text emails or attachments, whether with uuencoding or MIME