Back then they may have had relay boxes. You would case your mail for the route, put it in a bag and it would be dropped off in a green relay box.
You would then go to that box, take out the mail for your route and then deliver it.
Maybe back then they used regular mail boxes for relays.

In an age when autos/trucks were still relatively new and not everyone had a drivers license, maybe he was a driver. Possibly driving a truck to pick up mail from the boxes around town.

Maybe the D stands for Delivery, as that is what he appears to be doing: getting ready to deliver mail.

[Badges for letter carriers (aka"mailmen") were numbered. At any rate, he's not delivering, he's collecting mail - in this case packages - customers have deposited in the box. -tterrace]

With the "D" on his cap, I'm wondering if this might be a doorman or porter from a hotel or building, putting mail into the box.

[Only a postal employee would have access to the interior of the box. Also, his badge is an official Post Office Department issue - the last year of this wreath design, in fact. I haven't been able to find the significance of the "D"; usually it's a number, but others denote a specific function. -tterrace]

---

Washington Post, January 29, 1922.

Blizzard Costs 1 Life;
Capital Goes Afoot;
Business is Halted

More than 24 inches of continuous snow had last night covered the middle Atlantic section, with Washington as a center, to a depth of a foot to nearly 30 inches; caused the suspension of practically all business and social activity; disrupted transportation and shut most of the populations in their homes.

The storm, which weather bureau officials, after looking up their records, said was one of the most severe in history and exceeded in the depth of snowfall only by the long-remembered blizzard, 1899, was moving slowly last night up the coast from its position during the day off Virginia. …

Automobiles and other motor vehicles likewise were unable to cope at all during the day with the snow and last night the streets of downtown Washington were lined with abandoned cars, some of which owners had not been able to move since Friday night. Taxicabs did a thriving business, but as the snow increased their numbers were decimated by the drifts. …

Thousands of government employes walked to work and many in the outlying or suburban sections stayed at home. When noon came and the storm showed no signs of abating and the weather bureau held out no hope, many bureaus dismissed the employes so as to allow them to get to their homes.…

As an ex letter carrier I wonder if that was the standard winter uniform at the time.

Also what the "D" meant on his hat? I don't think he was a carrier as they would have had to take the train or bus to get to their routes, they would not have been collecting bags of mail.