It really is not easy to prove non-service. I read of a case where the tenant won because their door was decorated for Halloween and the server said it was clean.
My friend won their case because they were able to prove that they had never used that address for mail. Also, on the date of supposed service, they had family in town.
This was a roommate/subtenant case where clearly the PT was trying some dirty tricks by having everything sent to the apt. address and having a friend lie about service.
What I learned is in any rommate/sub-tenant agreement should be written that the mail box is shared.
I think they were lucky.