Validation using Excel VBA code

Hi Expert.. I have a sheet, where user have to fill data. There is a column Department.. user should not enter or paste a value other than defined departments. i have applied validation using data>>validation. but it works only if the user selects department from the validation list. but if he copy data from his source and paste it in my Sheet, then the validation doent work. i have created a button (Validation) on the top of the sheet, to validate the data. it validate for email id, contact numbers, name, etc... when the user clicks on this button it throws errors.. the same way it shold throw the error if the department is not in the validation list... please help me out... its urgent

Thanks in advance
Regards
Seema