Yes. The maid

The mail don't come on Sunday!!!

I agree with this analysis, however there are numerous times I don't check the mail on Saturday and don't look until Sunday (I have a PO box and just happen to stop when I'm in the area)

The police did not even bother questioning the wife herself, or looking for fingerprints, so yeah, screw that maid, I guess. Nice job, police.

I didn't understand this at first because I live in a flat in china and rarely get mail

Even if the mail came on Sundays, it wouldn't come until the afternoon (at least where I live). So if she was getting the mail on Sunday morning, then the simplest explanation is that she didn't get it Saturday afternoon and the detectives are really lazy and suck at their job.

NO POST ON SUNDAYS BITCH!