Postmaster Judy Wagner said the incident occurred about 1:45 p.m. Monday at the downtown building on West Fifth Street, which dates to the 1930s.
Mail continued to go out of the location Tuesday, but no one was allowed in the lobby, Ms. Wagner said.
Residents with post office boxes there were able to come around the back of the building to pick up their mail as long as they brought identification.
Carriers were moved to Jacksonville.
Ms. Wagner said U.S. Po-stal Service maintenance helped with clean-up, and an engineer will look at the building at a later time. The exact cause of the ceiling collapse was unknown as of Tuesday morning.
Anyone with questions can call the post office at 903-683-4741.