What is MDA (fetchmail) and MTA (sendmail)?

MTA - Mail Transfer Agent - sends and receives email between MTAs and among other elements of an email system

• Postfix
• Sendmail
• Exim

MDA - Mail Delivery Agent - receives email from an MTA for delivery to a mailbox

• procmail
• dovecot
• Courier

MUA - Mail User Agent - email client

• Thunderbird
• elm
• Exchange

Here is an overview of how the elements interact.