What is the defference between MTA and MDA (email)?

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

• exim
• postfix
• sendmail

MDA = Mail Delivery Agent
(receives email from an MTA for delivery to a mailbox)

• courier
• dovecot
• procmail

MUA = Mail User Agent
(email client)

• Thunderbird
• elm
• mutt

Note: An overview of how these agents interact.