Try each of the following steps in order until the issue is resolved:

1.  Exit and Sign Out of The Raiser's Edge
2.  Save the Merge to your workstation with a different name, rather than saving over an existing file.
3.  Recreate the parameter file for the mailing
4.  Run the mailing again
5.  Reboot the workstation
6.  Update/repair the workstation.
7.  If Microsoft Office 2003 and Microsoft Office 2007 are both installed on the workstation, uninstall the version that is different from that installed on the server
8. Grant the user the appropriate NTFS rights