Post problems in Mokotow (Warsaw)

Hi everyone. I'm new to Warsaw and this site - just relocated from London with my partner - and I have a bit of strange situation that I hope you might be able to help me with. It's my partner's birthday this week, and so far he's received two cards, etc. from the UK. The envelope of the first card was completely torn open; it was lying on top of the rest of mail when I took it out of the box and the card was at the bottom of the pile. Today, he received a second card, and this time the end of the envelope had been slit, as i a letter opener had been used. We live in Mokotow in what appears to be a quite safe building and the mailbox is locked, so I can't think how this is happening. Luckily there was nothing enclosed with the cards, but it's disturbing to think someone is opening our mail. Is this known to be a problem? Has anyone else experienced anything like this? Should we be reporting it somewhere? Is it possible to get a PO Box at the post office where the mail might be more secure?

Thanks for your help - it's such a bizarre situation. Apart from this, we're having a great time in Warsaw and really enjoying the city.