Outlook 2003 will accept html signatures named .html or .htm, Outlook 2007 apparently only likes them named .html.
The following bat file (which is also a handy example of a batch for loop) fixes this. Outlook only looks for signatures on startup generally, we’ve certainly only had consistent success through running it before opening Outlook. We’ve appended it to our logon scripts:
c: cd \ cd "%USERPROFILE%\application data\microsoft\signatures" for %%J in (*.html) do rename "%%~NJ.html" "%%~NJ.htm"