Mail Server works with other programs in collaboration  to make up what we know as Messaging System. Messaging system must work without any flaws to keep smooth movement of Inbound and Outbound emails. Mail System uses following protocols to send and receive email messages.

SMTP (Simple Mail Transfer Protocol) – For Sending emails

POP3 (Post Office Protocol 3) or         -  For Receiving emails.

IMAP (Internet Message Access Protocol)

At any point you found out that emails are not reaching their destination, there are high chances of these emails would get stuck in Mail Queue. Only way to release emails from mail queue is to restart mail services. Restarting mail services is done from Mail Server. Make sure before restarting mail service you ask anyone who has good knowledge of doing it.