USPS Outgoing Mailbox, normal mail?

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I was driving last night and saw a USPS and drove into the parking lot, I saw 4 large mailboxes outside and all of the had slots, and above them them it said, "Outgoing Mail" so I dropped my mail in there and left. I was thinking, is it OK for normal mail to be dropped in there? Or does it matter? I may just be being paranoid, but I was wondering is there some stipulation on dropping your mail off directly into the USPS mailboxes? Thanks.

USPS Outgoing Mailbox, normal mail?