Two ways to skin this cat, depending on the breed. (Yes, I'm into feline dermatomy!) You will have a button that somehow generates the letter you want, perhaps by running a report or by transferring text to a file. Either one could be built using the forms's control button wizard.
Case 1: You only want to know the LAST date on which a letter was sent to the customer. You NEVER want to know about any previous letters.
In that case, it is OK to link the form to the raw table. under the button's code, update the date field with the Date or Now functions. (Pick one.)
Case 2: You want to know EVERY letter you send to ANY customer.
In that case, you need two tables. One has the customer's info in it. The other is just the customer ID and the date of the letter.
Sub-case A: Make the date of letter transmission visible as a sub-form on the form that links to your customer table. Link the sub-form and parent form through the customer ID field. Place the letter-writing button on the sub-form. When you click it, add a new record with the parent form's customer ID and the Date or Now function.
Sub-case B: Write some code to open the letter history table as a recordset. Add one new record to the recordset. The customer ID can come from the form. The date can still come from the Date or Now functions.