Basically, the costume is a big inflatable suit that has a dog on it biting the backside.
However, the Royal Mail didn't see the funny side, as postal service workers do actually get bitten by dogs quite frequently.
So, despite Michael Nelhams just trying to amuse the people of his community during a difficult time, he's been told that he'll have to do it in a different costume to this one.
A few residents on Nelhams' patch in the Lincolnshire seaside village of Mablethorpe had alleged that someone had actually complained about the costume, but the Royal Mail has not confirmed this.
However, the company has been clear that he should find alternative ways to try to keep the public's spirits up.
A Royal Mail spokesperson told Louth Leader: "Over the last few weeks we have been inundated with messages and pictures from our customers across the country in praise of the extra lengths our postmen and women have been going during the coronavirus crisis to raise spirits, and money, for local charities.
"We know that local communities really value what our postmen and women are doing in these challenging times, and we're really proud of them.
"On very rare occasions, we have asked colleagues to show sensitivity in their choice of costumes.
"Last year saw 2,484 dog attacks on postmen and women across the UK, some resulting in serious injury."
Whether you agree or not, you have to admit, 2,484 attacks by dogs on posties in a single year is a shocking statistic.
Even though he's been told to rethink his morale boosting strategy, Nelhams says he'll continue trying to raise a smile from the folks on his rounds.
He confirmed: "I'm still going out in fancy dress, [but] not wearing the dog-biting-bum costume. I'm keeping the spirit of my customers and making many people smile in these hard times in lockdown."
Residents have broadly enjoyed his antics, and some condemned the alleged complainant and the Royal Mail as 'killjoys'.
Under a post on Facebook, one person said: "I can't believe someone has actually complained about that. What they should be doing is thanking them for doing a fantastic job."
Another added: "Why ruin it for the kiddies and those of us who love to see a bit of happiness in our village?"