Royal Mail said addresses in Alton Road and Belvedere Parade in Bridlington, East Yorkshire, have not received mail "on some occasions recently".
"These kinds of attacks do occur at this time as the gulls are nesting and rearing their chicks," it added.
Chris Collett, of the RSPB, said: "Gulls might come across as more aggressive as they defend the chicks."
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The Royal Mail said postal workers were attempting to deliver to customers every day but safety was paramount and "swooping attacks made it difficult".
Mr Collett said most of the attacks at seaside towns are by the herring gull, although other species of gulls like kittiwakes can also be aggressive to protect young.
"It can be a bit scary so try to avoid nesting areas, the worst time is the few weeks when the chicks are getting a bit older but before they fledge."
He added that gulls have become more common at the seaside because the flat roofs provide a safe nesting area and there is a ready supply of discarded food.
In September, a council report said 22 people in nearby Scarborough, Whitby and Filey had been attacked by gulls during the previous six months.
Both herring gulls and kittiwakes are protected under the Wildlife and Countryside Act.
Postal deliveries in parts of a seaside town are being disrupted by "swooping attacks" by seagulls.