Royal Mail issued its daily service update, with 56 postcodes set for slower than usual deliveries.
The postal service has said both Covid-related self-isolation and high numbers of workers off sick are affecting delivery times.
Areas affected include Pontefract WF7, WF9 and WF11.
Royal Mail apologised for the inconvenience and said it plans to "rotate deliveries" to minimise the delays - meaning some areas will temporarily see slower posting times.
A statement on the Royal Mail site reads: “Deliveries are operating as normal across most of the country today. We aim to deliver to all addresses we have mail for, six days a week.
“In a small number of local offices this may temporarily not be possible due to local issues such as Covid-related self isolation, high levels of sick absence, resourcing, or other local factors.
“In those cases we will rotate deliveries to minimise the delay to individual customers.
“We’re sorry for any inconvenience and thank you for your understanding.”