It could be that the email service is unavailable - check with other colleagues whether they can use email, check service status, or contact IS.Helpline@ed.ac.uk for information.
Another reason could be that your mailbox has reached its size limit. If you don't tidy up your mailbox before you reach your mailbox limit, the Exchange system will automatically stop you sending mail. At this point you must tidy up your mail. If you continue to ignore the warning messages the system will eventually prevent you from receiving mail as well. Please see the following article:
This article was published on Aug 22, 2013