Run-time Error 2293

I have an access database that sends emails. This database works fine
in Office 2003. Recently we upgraded 3 PCs to office 2007 2 of those
PC work fine but on one of them I get the following error when trying
to send the email:
Run-Time error ‘2293’:
Microsoft office access can’t send this email message.

If I set Outlook Express to be the default mail editor everything
works okay. It also works if outlook is closed. I have also tried
office diagnostics, and reinstalling. I’m not sure how to begin
troubleshooting this. I have read the KB:
http://support.microsoft.com/kb/263084/en-us

This KB was not helpful in solving this issue. Any ideas as to why
this workstation is not functioning?

Workstation #1: Vista SP1 / Office 2007 SP1 (working)
Workstation#3: VMware virtual machine / XP SP3 / Office 2007 SP1
(working)
Workstation#2: XP SP3 / Office 2007 SP1 (not working)

Here is the Access code we are using. I don't think this has anything
to do with it because other 2k7 workstations are not having
challenges.
**********Start **********

DoCmd.SendObject , , acFormatRTF, TechnicianAssigned, , , "A job has
been assigned to you for " & FirstName & " " & LastName & " Job
Number: " & JobNumber, FirstName & " " & LastName & vbCr & "Challenge:
" & ProblemDesc & vbCr & "Work Last Done: " & Forms!
JobContinuation!.ProblemResolution & vbCr & "Job Created: " &
DateTimeStarted & vbCr & "Location: " & Office & vbCr & "Work Phone: "
& WorkPhone & vbCr & "Job Number: " & JobNumber & vbCr & "IP Address:
" & Forms!JobContinuation!.IPAddress & vbCr & "GBFA Number: " & Forms!
JobContinuation!.GBFANumber & vbCr & "Quick Reference Number: " &
Forms!JobContinuation!.QuickReferenceNumber, True, False

**********End**********