Hmmmm... maybe ask them to send you their bank account details so that you can verify who they are before sending them your information... and maybe they'd be interested in helping transfer $50,000,000 (FIFTY MILLION DOLLARS US) from a secure account in Nigeria that your great uncle Sigmund left upon his death, you'd cut them in for a small percentage of course.

Normally even asking someone where you can download their public GPG/PGP key results in a "youwha?" type response.

The difference with snail and electronic mail is where liability rests - you post someone sensitive documents and the postie (or anyone else) gets caught opening it and they're in trouble - you email someone sensitive documents in an unencrypted format and you'll probably just get told it was your fault for being a plonker.