For all out there, who are getting the same issue, here is the solution:
Preventing "Adding Entry will cause text list to exceed 64k" errors
How to stop this happening
To stop the server expanding the groups before sending the mail (and
hence, stop this error message), you need to add a ExpandPersonalGroups
field to the document being emailed, and set its value to zero.
This really is as simple as just putting doc.ExpandPersonalGroups = 0 in your code, where doc is the NotesDocument object being mailed.
If you're putting the field into a form, the field needs to be of type text and computed when composed.