[dba-Tech] Outlook custom forms

I don't fully understand the Send Form Definition With Item option when creating a custom form in Outlook. This is what I think the documentation is saying: 

1.) You must send a custom form's definition if you're sending the form across the Internet to someone not on your Exchange server. 

2.) If the custom form runs code, you can't send the form definition. 

Now, I don't really know what #2 means. I want to create a custom Message form that uses code to fill in a few fields before sending the message. I want to send the custom Message to recipients not on an Exchange Server. Nor am I running Exchange. Does #2 just mean I can't. Or, does it really mean that I can run the code to fill in the fields on my end, but that the recipient can't. The latter seems logical, but I just don't know. Of course, all of the above confusion is contingent on #1, if I understood it correctly. 

Susan H.