sendmail

Sendmail is the most popular UNIX-based implementation of the Simple Mail Transfer Protocol (SMTP) for transmitting e-mail. When a sendmail server receives e-mail, it attempts to deliver the mail to the intended recipient immediately and, if the recipient is not present, it queues messages for later delivery.

Sendmail is used to deliver or route email. When you enter an SMTP server, when you config your mail client, you are most likely pointing to sendmail or another Mail Transfer Protocol.

However, since it does not provide a mailbox facility and for other reasons, other software such as a POP3 or Internet Message Access Protocol server is also needed. Most Internet service providers (ISPs) provide both an SMTP server (such as sendmail) and a POP or IMAP server.

When using POP, it will retrieve messages from your main Inbox folder only. The POP protocol does not recognize any other folders in your mailbox. Most email services and ISPs use this type of server, especially for personal email accounts. They hold incoming email messages until you check your email, at which point they are transferred to your computer. Messages are typically deleted from the server when you check your email.

IMAP Internet Message Access Protocol (IMAP) servers. These servers let you work with email messages without downloading them to your computer first. You can preview, delete, and organize messages directly on the email server. Copies are stored on the server until you delete them. IMAP is commonly used for business email accounts, but is increasingly common for personal email as well.

When using IMAP, email messages will remain on the Exchange server, but messages are able to be viewed on your IMAP client. IMAP does support multiple mail folders. IMAP does not support calendar, tasks, or contacts. IMAP essentially gives you access directly to the email server. By default, any message you delete is deleted from the mail server and will not be found in Outlook on campus or in Outlook Web App.

Simple Mail Transfer Protocol (SMTP) servers. This outgoing mail server handles sending of your email messages to the Internet. An SMTP server handles only the outgoing email, and is used in conjunction with a POP3 or IMAP incoming email server.