I suspect that simply reading a postcard whether snail mail or e-mail is not tampering with. Opening, destroying, altering a letter is a crime for snail mail. The outside of a snail mail letter is also public information, so at a minimum “they” know who you are writing or who is writing you.
I also suspect if you refuse to accept the new terms and conditions for using a free email system, you will at some point lose that ability. Their service, their rules and if you choose not to accept their rules why should they provide you with free email?