If changes have been made to the back servers, then sometimes the cached email address are incorrect.  

First you may need to delete the cached address.  To do this follow this link >> http://bible.mirrorsphere.com/DOC-725199378841833-1728622369603823

1.
  1. Open a new email
  2. In the From Box (See this guide if it is not visible http://bible.mirrorsphere.com/DOC-725199378841833-1360559022522511) click on the Drop down icon
  3. Then click on Other Email Address..
  4. On the next Dialogue box Click on From 
  5. Now Click on the Drop down under Address Book
  6. In the drop down , Click on Global Address List 
  7. Now you will see the list, but under Address Book it will show Global Address List - your@emailaddress.com
  8. Find the account you want to send as, Select it and then Click  on OK
  9. You will see something like this, then click on OK 
  10. The address is now in the From  field, now send the email as normal