>>The message is "The custom form could not be opened. Outlook will use an Outlook form instead."

Whenever I get an error message that I don't understand, I type it into Google, nearly everytime you'll find someone else with the problem.
http://www.google.com/search?as_q=&num=10&btnG=Google+Search&as_epq=Outlook+will+use+an+Outlook+form+instead&as_oq=&as_eq=&lr=&as_qdr=all&as_occt=any&as_dt=i&as_sitesearch=&safe=off

Here's one of the results for it
http://support.microsoft.com/support/kb/articles/Q223/4/81.ASP

Could be teh encrypted message scenario, its the only one that makes sense. You might wanna try that custom alerts thing.