The sorting office in Peacehaven’s Hoyle Road had a leaking roof on Friday after the building was battered by heavy rain and strong winds.
When the roof was assessed, contractors found it was more damaged than they originally thought.
David Wallach, who lives in Peacehaven, said he was told to expect significant delays to post due to the damage.
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A Royal Mail spokesman said: “We can confirm there was a leak in the roof of the delivery office last Friday following the severe weather conditions. Certain deliveries continued but some were affected.
“The roof has since been assessed and due to the damage and the length of repairs necessary we’ve temporarily moved the operation to our delivery office in Brighton.
“We have been notifying customers of the change and are working hard to limit any further disruption.”