If the creator of the Donor Acknowledgement Letter unchecked the option that other users can modify this mailing when the mailing was saved this message will appear.  

To correct it, do the following:
1. Have the user who created the Mailing right click on the mailing and select Rename
2. Give the mailing a new name and check the box that Other users may modify this mailing
OR
1. Have the user who needs to make the changes open the mailing
2. Click File>Save As and give the mailing a new Name
3. The user will be the owner of this mailing and will have rights to modify it.