9.25. Attach Plus is attaching winmail.dat files
Attach Plus appears to be attaching a 'winmail.dat' file to outgoing emails. This often causes problems when the recipient is on a Mac.
Attach Plus isn't the one attaching the 'winmail.dat' file to the outgoing email. Basically, that is Outlook doing strange things. This Microsoft article describes what is happening: http://support.microsoft.com/kb/278061.
The suggested resolution is to change mail format to either HTML or Plain Text.