From what I can tell, the headers "To" and "Subject" are not mandatory. They however have defined values (of "") inside the "email_message_class" class.
When you are sending a message to many BCC recipients, you don't really want to define a "To" field as that would confuse the recipients. If you do not defined a value, the class will use the empty string, which will cause an empty new line to be added in the message's header. This will cause the e-mail clients to dump the message's source for the readers, as they will think that the new line represents the ends of the headers list.
It took some time to trace this problem, so I hope it helps others who are bothered by it.