Households and businesses in Borehamwood will receive post at different times after changes to delivery practices.
Royal Mail is updating routes and schedules as part of a national modernisation plan, and says postmen and women will continue to deliver in the morning and for longer during the day.
The company says many customers will continue to get their post by lunchtime, but the time customers receive it will depend on where they live on the new routes, and times may also change due to varying mail volumes.
The company says the changes are in order to adapt to the growing number of parcels customers are receiving, which will be delivered by vans and trolleys.
It will be writing to every address in the Borehamwood postcode area about the changes it is making.
If customers have any queries after they have received a letter, they can call on 0845 601 1399.