As noted, the only real risk if you are mailing a payment to a business is if you leave the letter in your own rural/suburban type mail box where people might steal it, having stolen it they may "wash" the check and re-write it for more money and to themseves. There is a specific type of pen on the market that's resistant to this scam, called a "gel pen". More info at this link:
Check Washing - WHAT IS IT?
If you are mailing a check to an individual, there is some risk of the check being stolen from their mail box as well. That said I have sent and received checks via USPS for many years with no problems.