The Postal Service confirmed that there had been three confirmed cases at the site in total.
In November 2020, the service confirmed its first positive case of coronavirus in the office – meaning there have been two more cases since then.
The service says there have only been three confirmed cases since the start of the pandemic.
A staff member has contacted Lincolnshire Live to express concerns about the number of people isolating themselves.
Royal Mail has confirmed that a number of people are self-isolating after being contacted through testing and tracing, but said it could possibly come from outside sources.
A Royal Mail spokesperson said: ‘Throughout this crisis, every decision we make puts the health of our employees and customers first. We have implemented a series of preventive measures to protect our customers and colleagues.
“We were the first UK company to implement social distancing measures for parcel delivery.
“We don’t temporarily hand over our wearable devices to customers to capture signatures.
“In addition to encouraging good hand hygiene, standard working methods have been revised to ensure that colleagues maintain appropriate social distancing at all times.
“All staff have been informed of the social distancing measures agreed jointly by local management and the CWU. This was supplemented with visible reminders such as posters and one-way markings on the ground. ”