This is exactly why the USPS started replacing the blue box opening slots with a narrower version in New York City and surrounding areas starting back in 2019 [1], to prevent "mail fishing" -- retrieving envelopes from people paying bills, specifically to wash found checks in order commit fraud.
Which, if you used to use the blue boxes for mailing thin packages, is a huge annoyance because you can't anymore. :(
Seriously, these days nobody should be writing personal checks using a pen. If you do need a check for a transaction, use a cashier's check (different from a personal check, it's drawn on the bank) for occasional large amounts (like apartment deposit or buying a used car), or most banks allow you to send a paper check via online banking for routine needs -- you fill out the details online and it mails a paper check to the recipient that, AFAIK, can't be washed for check fraud. (All of this is USA specific, to be clear.)
Which, if you used to use the blue boxes for mailing thin packages, is a huge annoyance because you can't anymore. :(
Seriously, these days nobody should be writing personal checks using a pen. If you do need a check for a transaction, use a cashier's check (different from a personal check, it's drawn on the bank) for occasional large amounts (like apartment deposit or buying a used car), or most banks allow you to send a paper check via online banking for routine needs -- you fill out the details online and it mails a paper check to the recipient that, AFAIK, can't be washed for check fraud. (All of this is USA specific, to be clear.)
[1] https://www.nytimes.com/2019/03/21/nyregion/mailbox-theft-fi...