The phrase "the transom" is correct and can be used in written English.
It refers to the horizontal beam or bar that separates a door or window from a wall above it. It can be used in a sentence as follows: "The sunlight streamed in through the transom, illuminating the room with a warm glow."
Dictionary
the transom
noun
A crosspiece over a door; a lintel.
synonyms
Exact(54)
"Things kind of came over the transom".
"Things kind of came over the transom," he explained.
I dropped my cartoons off at the transom.
Nicholas Music Inc". in rusty gold paint on the transom.
"For 'over the transom.' " The papers of U.S. Supreme Court Justices enter the office of history over the transom, too, if they come in at all.
Over the transom at Parterre: Plácido Domingo's baritone turn in Berlin, Jonas Kaufmann's Lohengrin in Munich.
Last week, no fewer than three blockbuster marriage proposals came through the transom.
Similar(4)
It "started with an over-the-transom letter, an anonymous letter, alleging collusive bidding at Pier 57," Mr. Morgenthau said.
"If this woman had come to us over-the-transom, I would have been exceedingly dubious," Bridis said.
"If something comes in over the transom...how do you vet that the information is legitimate?" Keller said.
On the off chance something comes in over the transom...scratch that; I'll delete the email account.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com