Definition of MTA / MDA
A mail server consists of at least 2 processes: the MTA (Mail Transport Agent) and the MDA (Mail Delivery Agent). The MTA is the SMTP server, responsible for receiving e-mail messages. The MTA then transfers the received message to the MDA, which stores it (in the right place) for the user to retrieve (via POP3, IMAP4 or other).
All mail servers on the market integrate an MTA and an MDA: postfix, exim, qmail, exchange, lotus, … One part to receive e-mails and a second to store them for delivery to users.
ALTOSPAM is a filtering SMTP relay, and as such is an MTA that filters emails and relays them to another MTA.