Step 1: Understand why mail is date-stamped:
Every letter that reaches an office is stamped with the date of receipt.
This is done in the mail inward section.
Step 2: Find the purpose:
The stamp acts as proof that the organisation got the letter on that day.
It helps the office to acknowledge receipt, track delays and answer within time.
So the missing word is 'acknowledge'.
Step 3: Related practice:
After stamping, the mail is entered in the inward mail register.
The date also helps in replies and in later disputes about delivery.
Final Answer:
The blank is filled by acknowledge (that is, to record or confirm receipt).