The director of an Oswestry business believes his property is being used as the headquarters for a local gang, having battled graffiti and petty theft in the past.
But now the property is littered with remnants and contents of opened letters from post boxes in Oswestry.
It comes amid Royal Mail strike action, with no collections on Wednesday and Thursday of this week, leading to post boxes in the town reaching capacity.
A director at Catalogue Surplus Centres Ltd on Willow Street said: "The post boxes are overflowing and people are stealing Christmas mail to find cash.
"This could be UK-wide, as the post boxes aren't being sealed when they're overflowing.
"It's incredibly un-Christmassy of them to do this. Opportunists are sticking their hands in to find things. I chased one of them off last night."
The director is upset that thieves have targeted festive mail, as people send presents, money, and cheques to their loved ones on the run-up to Christmas.
However, important items of post have also been littered on the property after being stolen from post boxes - including cheques for medical insurance.
He added: "Debris, insurance documents, and bits of cheques are covering our flat roof - this is important mail that people are missing.
"We've found cheques for medical insurance. One of my friends sent a cheque for medical insurance in the post and it got lost.
"Because of the missed payment, the medical insurance became void, and when she rung up to say she'd been diagnosed with cancer, they refused to cover the actual treatment.
"So if you've sent important documents in Oswestry, please check that they've actually arrived."
A spokeswoman for West Mercia Police said: “We have received a report this morning (Friday 2 December) of a theft from a post box on Willow Street in Oswestry.
“It is believed the theft took place at around 5.30pm yesterday (Thursday 1 December). Investigations are ongoing and anyone with any further information is asked to contact police.”
Royal Mail is asking its customers to post their mail earlier than usual for Christmas this year as a result of the ongoing strike action
Further strikes organised by the Communication Workers Union (CWU) are set for the following dates, including the two days before Christmas Day:
Friday, December 9
Sunday, December 11
Wednesday, December 14
Thursday, December 15
Friday, December 23
Saturday, December 24
Royal Mail has been contacted for comment.