Hi Matthew,
Thanks for reaching out here. LL21 is a bit outdated, so there might have been changes to both Outlook and Windows in the meantime. If the exe is registered fine, we should be able to see a bit more if you logged an attempt to send a mail. See here for a guideline how to create such a log file. Also, you might try to use XMAPI instead of MAPI. Feel free to post a log file here (provided it doesn’t contain any personal data you’d like to protect) or otherwise create a case in our support center here. It might also be worthwile to check with the latest trial if LL25 works as expected. We’d be happy to provide you with a favorable update offer if that would solve the issue for you.