We often heard about IMAP and sendemail.
so what's the difference between them?
In high-level terms, sendmail is an open-source mail server application. IMAP, or Internet Message Access Protocol, is a protocol (i.e. a formal specification) for how email clients receive mail from a server (like sendmail, but there are many others). Specifically, IMAP servers keep email messages on the server, and the mail client downloads copies. The IMAP protocol defines how the client and server should keep them in sync.
Think of IMAP as a rulebook. sendmail and other email servers implement the server's set of rules, and email clients implement the client's set.