If your user complain that they cannot send e-mail through your mail server, this can mean that your mail server is overloaded and cannot cope with the amount of received messages. This can happen when somebody is sending spam through your mail server.

To return your mail server to an operable state, you will need to delete  unwanted messages from the mail server's message queue.

To see the messages in the message queue and to delete them:

• Total number of undelivered messages. When messages come to your mail server they are first added to the main queue. Then, the mail server preprocesses them in order to find out whether they should be delivered to a local e-mail account on the same server or sent further to a remote recipient's e-mail address. After preprocessing, the messages directed at local mail recipients are put to a local queue, and the messages directed at remote recipients are put to a remote queue. Once delivered, the messages are removed from the queues.
• Message properties: subject, sender, recipient, queue type (local, remote, not preprocessed), date the message was sent from user's computer, the time lapsed since the moment when message was put to queue (age), and message size.