The ballot goes into an envelope. You sign the envelope. That envelope goes into a regular postal envelope. When the ballot is received, the signature on the inner envelope is compared to the signature on file, and, if accepted, the envelope is opened and the ballot is transferred to a box to be counted.
Then the ballot is placed in the same hoppers that the in person votes are put in and they are commingled.
The outermost envelope is verified and opened, and the innermost envelope is collected with all of the others in a different area for counting.
My town has a bunch of dropboxes which are picked up by election volunteers if you still don't trust the mail. An election volunteer could "lose" a box of ballots but that seems like it would be pretty easily detectable as well.
Since mail tampering is a federal crime maybe Trump could pledge to pardon every mail carrier who throws away ballots from likely Biden voters. Seems pretty unlikely, but I don't know how to counter that.
To do the correlation in person you would have to do it in the polling place, on election day, in front of election monitors and the voter, during the 30 seconds between when the person fills out their ballot and when they drop it into the box.
If your name is on the envelope, now it can be done anywhere between your mailbox and the polling place during the period of days or weeks between when you put it in your mailbox and when it gets counted. Wouldn't we then need independent election monitors guarding the incoming mailboxes 24/7 for several weeks?
For reference, here is how it works in Washington state: https://www.sos.wa.gov/elections/faq_vote_by_mail.aspx
Yes, the mailman could commit a federal crime and figure out how you voted for. So if you're worried about that, drop the envelope in the actual ballot box.
The mailman sees even more. If you live in a condo and drive a Prius with a "Coexist" sticker, you probably voted one way. If you live in a single-family home with a big yard and you drive a pickup with a "Blue lives matter" license plate holder, you probably voted the other way.
Your party affiliation is already public record, no need to attempt to infer your preference from your name or address.
I still don't understand how voting in person is supposed to be any more problematic than e.g. buying groceries in person. Wear a mask, stand six feet apart etc., what's the problem?
Grocery stores don't have checkout lines that are hours long where you are standing around near the same group of strangers for an extended time.
Worse, the states with the longest, slowest voting lines also tend to be the states with less stringent mask requirements, less diligent enforcement of the requirements they do have, and more people who ignore the requirements.
So you are suggesting that the major political parties would sanction, even informally, having people go out to houses to illegally harvest ballots, or throw out ballots they don't agree with? To get one nearly worthless vote at a time, with a very possible multi-year prison sentence for each. That is utterly ridiculous.
There have been irregularities as a result.
I can't imagine many votes have been discarded, that sounds more like a conspiracy theory. It wouldn't take long to get caught, since voters can easily verify their own vote was recorded.
This process keeps your vote anonymous.