In computing, a server is a computer program or a device that provides functionality for other programs or devices, called “clients”. This architecture is called the client–server model, and a single overall computation is distributed across multiple processes or devices. in computing System, there have different types of E-mail server : POP3, IMAP, SMTP, NNTP, LDAP, etc.
Post Office Protocol 3 (POP3) servers hold incoming e‑mail messages until you check your e‑mail, at which point they’re transferred to your computer. POP3 is the most common account type for personal e‑mail. Messages are typically deleted from the server when you check your e‑mail.
Internet Message Access Protocol (IMAP) servers let you work with e‑mail messages without downloading them to your computer first. You can preview, delete, and organize messages directly on the e‑mail server, and copies are stored on the server until you choose to delete them. IMAP is commonly used for business e‑mail accounts.
Simple Mail Transfer Protocol (SMTP) servers handle the sending of your e‑mail messages to the Internet. The SMTP server handles outgoing e‑mail, and is used in conjunction with a POP3 or IMAP incoming e‑mail server.
Windows Mail also supports additional server types: Network News Transfer Protocol (NNTP), which is used to read and post newsgroup messages, and Lightweight Directory Access Protocol (LDAP), which is used to access directory services.