Email Services
The process of getting an email from one person to another over a network or the Internet involves many systems working together. Each of these systems must be correctly configured for the process to work. The sender uses a Mail User Agent (MUA), or email client, to send the message through one or more Mail Transfer Agents (MTA), the last of which will hand it off to a Mail Delivery Agent (MDA) for delivery to the recipient’s mailbox, from which it will be retrieved by the recipient’s email client, usually via a POP3 or IMAP server.
On Ubuntu, Postfix is the default, supported MTA.