Email address selection from Outlook drop list is sending to users logins not to thier email addresses. eg. type A in To field it shows all users with A I selected me then it email to my login

I am receiving a lot of undeliverable emails - I have checked whats really going on there - I saw that from Microsoft Outlook 2003 when I click on New in To field I type just A there is a drop box list opens up (which was not like this before) it has list of all names from Global Address List starting from A like my name is Azfar Lashari <azfarl@mycompany.net> but our company format for EMail Address is  FirstName.LastName@MyCompany.Com  and when I select myself from that Drop Box List it is then going to email Azfarl@mycompany.com which goes to undeliverables and Undeliverable messages goes to Exchange Administrator which is me so same like that I am now receiving these kind of emails alot. Please help what should I do to resolve the problem.