Post has been delayed in Hatfield, Welham Green, Brookmans Park and Essendon.
The Royal Mail has said it's "very sorry" that "resourcing issues" at its Hatfield Delivery Office have caused the delays.
This will likely affect deliveries in parts of AL9 and AL10 areas this week, and into next week.
Royal Mail added: "We apologise for any inconvenience this may cause and thank you for your understanding. We're working to resolve this issue and return to normal service as quickly as possible."
The delivery service added: "Please note: To collect an item from the Customer Service Point, you will need a valid 'something for you' card, advising we're holding an item for you there.
"For our Customer Service Point opening hours, please see royalmail.com/services-near-you as they may have changed."
Royal Mail’s online customer service team has previously warned of “coronavirus-related absences” affecting its services.
And last month issues in Stevenage led to residents across the town not getting their post.
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