Someone posted a problem with Lotus Notes 8 working with XpressMail, but I was unable to reply to the thread. I had the same issue and did some testing and found a workaround.... so if anyone is interested here is what I did:
This involves editing the registry so I would expect only those users who are familiar and comfortable doing something like this to continue..
First I exported the "HKEY_LOCAL_MACHINE\SOFTWARE\Lotus\Notes\8.0" key, which of course includes the datapath, name, description, and everything else underneath the key, etc. Next, I edited the registry file with a text editor, changed "8.0" to "7.0" and then imported back into my registry. This simply created a new "HKEY_LOCAL_MACHINE\SOFTWARE\Lotus\Notes\7.0" key with the same information. After that, XpressMail magically worked again.