Reasons:
SMTP is a Simple mail transfer protocol used to send mails from User to mail server using some mail extension protocol in order to support different type of email : text,audio,video etc (MIME is one that is used more)
I mentioned above that is used to send mails from users to mail server that`s why its called as push protocol.
and to access mails from mail server we need pull protocols.
Which are POP3 and IMAP and most often the later is used because of the property of accesing the mails without downloading which is not available in the case of first.
(A) POP3 is used for push and pull the mails from server.
(B) IMAP4 is used for push the mails to server.
(C) Without MIME, SMTP can also push all kinds of messages to the server.