The USPS has updated how and when mailed items receive an official postmark. Historically, mail dropped in a residential mailbox, a blue USPS collection box, or inside a post office was typically postmarked the same day.
Under the new process, same-day postmarks are no longer guaranteed unless an item is taken directly to a post office counter and a customer specifically requests a manual postmark. Mail deposited in drop boxes or mailboxes may not be postmarked until it reaches a regional processing facility, which could be one or more days later.