When a landlord is doing a credit check, you can also send your SSN with some peace of mind.
"That being said, you should always be careful. We recommend you do not send any seriously sensitive information through Ghostmail and refrain from corresponding with people you do not trust."
That being said, congrats on building something. I still haven't gotten that far.
While most email systems communicate over encrypted protocols, very few encrypt their data store.
For security's sake, please never send your SSN over email for any reason.
[0] Your SSN is your Primary Account Number for the United States.