It sounds like the Live Cycle Designer has a hard coded Mapi32.dll reference, and is not using the correct process to determine which mailer to use. If you are able to experiment in your environment, try the following:
o In Windows/System32 rename Mapi32.dll to something else (like Mapi32.dllx)
o Copy Mapipm.dll into Windows/system32
o Rename Mapipm.dll to Mapi32.dll in windows/system32
o Try your Submit sequence. Hopefully Pegasus Mail will popup. If so, complain to vendor
o Check for activity in your Mapipm.log.
o Reset your Dll by erasing Mapi32.dll in windows/system32; then rename back in your windows/system32 your Mapi32.dllx to Mapi32.dll