Sometimes an email address is entered incorrectly in the To field in an email and, if it has been used to send an email, it will appear in the drop down list when an email address is being entered after that point.
To delete it, highlight the incorrect email address in the To drop down list and select the 
to remove it.
Alternatively, highlight the email address and use Ctrl+Delete.
If you don't see the cross at the end, this means you will need to fix the email address at the cardfile contact level.
Further information
Create a Job, PO or Quote from Email
Diagnosing Incoming Email Issues
Diagnosing Outbound Email Issues