Writing letters is almost, but not entirely, like writing emails. Since messages travel by relay, messengers do not necessarily know the origin of their despatches, so it's a good idea to include a return address if you expect a reply. Similarly, it's considered decent manners to date your letters, since it helps (allies especially).
While letters are possibly the primary means through which you can roleplay, perfectly perfunctory letters are also fine. You can be as concise as you wish. Letterwriters in the 18th century were very wordy; that doesn't mean you have to be. However, you do have to write your messages yourself. Unlike ChatGPT you can't prompt me to write your missives for you.
As an example:
From: Duke Generic Example To: Marchioness France Farfelu d'Imaginaire, Hauteville Writing south of Nearby on the 20th of April, 1698 Dear Marchioness, Send dudes! Signed, His Grace the Duke Generic Example of Exemplia
I really should've chosen a better example. but assuming the message takes twelve days to arrive, the reply would look like this:
From: Marchioness France Farfelu d'Imaginaire To: Duke Generic Example, outside Nearby Writing from Hauteville on the 2nd of May, 1698 Your Grace, Please find attached the requested pictures. I have commissioned them especially for you. Signed, Marchioness France Farfelu d'Imaginaire
To which the Duke would find himself possibly disappointed on the 14th of May.