MTA - Mail Transfer Agent (post office)
MDA - Mail Delivery Agent (post box)
MUA - Mail User Agent (post man or people who retrieving the mails from postbox)
(Schema)
To keep everyone from checking other users' emails, MDA is protected by a user name called a login and by a password.
When an organisation's email server is improperly configured and allows third-party users on any network to send emails, this is called an open relay.
CONCEPT
Email is based around the use of electronic mailboxes. When an email is sent, the message is routed from server to server, all the way to the recipient's email server. More precisely, the message is sent to the mail server tasked with transporting emails (called the MTA, for Mail Transport Agent) to the recipient's MTA. On the Internet, MTAs communicate with one another using the protocol SMTP, and so are logically called SMTP servers (or sometimes outgoing mail servers).
The recipient's MTA then delivers the email to the incoming mail server (called the MDA, for Mail Delivery Agent), which stores the email as it waits for the user to accept it.Some comman MTA's in Unix based Operating system
- exim
- sendmail
- postfix
- qmail
- courier mail server
- zimbra
- binmail, the MDA part of Sendmail
- fdm - A combined mail retrieval agent and mail delivery agent
- maildrop
- postdrop
- postfix-maildrop - A mail delivery agent for use with the postfix mail transport agent
- procmail
- courier-maildrop
- dovecot - A mail server suite that includes a mail delivery agent
Graphical:
- Kmail
- Courier
- Mozilla Thunderbird
- Opera Mail
- Alphine
- Mutt
