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) people get a rough idea of how far away you are.

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.