Something really confuses me about the whole legal aspect of confirming you sent someone mail.
USPS has different types of mail (e.g. Certified Mail) for "proving" you sent your mail (e.g. tax returns).
(Note: This question is not specific to tax returns. I'm just using that as an illustrative example.)
My question is, (how) does this prove anything about what you actually sent?
I could very well just stuff an envelope with empty pieces of paper (or, more plausibly, something totally irrelevant that is likely to be discarded) and mail it to some address, then get every kind of confirmation possible for it from the post office. And then claim that I sent some important piece of mail (say, my tax returns, or a contract, or some other legally important document).
And when court day comes, I'll have all the "proof" I could possibly have from USPS for it, yet I clearly didn't need to actually do what I claimed.
So my question is, whether these mail certifications actually mean anything in a court?
If they do, then why or how -- clearly I didn't need to send what I claimed?
If not, then why do people need to use them for legal documents? What useful thing does it establish?