Possible Clues:

• The adders have a twisted tail, as you'll see on the envelope
• One who is named for the post
• Intended recipient
• Someone seen by postman looking through a window?
• One likely to have letters to his name
• I may not apply, but I ought to get the post
• The person to whom letter is delivered
• The Sir in 'Dear Sir'?
• One receiving a woman's garment in a D
• Name on a magazine cover