How do you know the zimmerman note was a reliable source

The main actors in the transmission of the message were tracked, followed  and then authenticated. The British intercepted the message on the British and American transmission lines as a coded message. They partially decoded it and understood the ramification and assumed the next level of transmission would require some kind of commercial carrier in Mexico. They tracked this down and conveyed all related information to the Americans who were inflamed and began their participation in WWI.