Mail can arrive at the post office by trucks a few hours or so before they actually open.
Then the mail is unloaded from the trucks and sorted which in some post offices can be as early as 7:00 AM to 8:00 AM
Then it can take until 3:00 PM for all mail to be sorted and delivered to the post office boxes and also could take until later in the day for the mail to be delivered to rural addresses depending on the mail carriers route, weather, traffic etc.
Mail carriers leave the post office between the hours of 8:00 AM to 11:00 AM depending on how much mail they have and the location of the post office.
Some mail carries may not leave the post office until around 12:00 PM but most do leave the post office with the mail to be delivered before 12:00 PM.
There's not set time that the mail carrier leaves the post office but they do so as soon as possible.
Then weather, traffic etc can factor into the time it takes for the mail carrier to deliver the mail to your address.
Mail leaves the post office to be delivered to houses, rural addresses etc between the hours of 8:00 AM to 11:00 AM depending on how much mail they have to sort and load.
There's no specific time for the mail to leave the post office and get on the road to be delivered.
However most mail does leave the post office for delivery by 11:00 AM
Then it can take a few hours or even until 4:00 PM before all mail is delivered and the mail carrier then returns to the post office.
Mail is also usually picked up at the post office to send off to the sorting facility by 4:25 PM
When mail gets to the sorting facility the mail at the sorting facility is constantly being sorted and sent out on trucks 24 hours per day 7 days per week.
This is to ensure your mail and packages gets to your address on time.
Most post offices open by 8:00 AM and close by 5:00 PM depending on the location.
But some employees might be at the post office before 8:00 AM to get mail off the truck and into the post office so that it can be sorted and delivered.