Possible Clues:

• Regrets deposit gone astray for Royal Mail service?
• Regrets depot is needing to reform mail service
• Blushing when grasping meaning, before paling: it's guaranteed to get to you?
• Entered after sign for this?
• Mail sorted with prestige, perhaps?
• Before Pole became aware of a means of communicating more securely
• Enrolled to get job in the royal mail
• Special form of delivery
• Specially secure delivery, but noted tops were off
• Type of mail procedure getting letters of prestige sorted specially