Mail transfer agent
A Mail Transfer Agent or MTA is responsible for transfer of mail from one server to another. Mail transfer starts with a DNS request for the MX-record of the destinations EMail-address. If the destination-mail server was found the MTA will transfer the message to de destination server.
You may want to use fetchmail to get the mail from another server.
procmail is there to process this mail, e.g. feed it to SpamAssassin.
sendmail sends your mail to whatever e-mail-address you like.