If it says Guarantor address on the envelope? no, it's not illegal to open. If it's in his name, yes.

However, since you already opened the letter, is your name or social anywhere on that paper indicating that he used you as a co-signer? If so, you have every right to be involved. Call the bank immediately and tell them this is fraud.

If not...then you can still call the bank and tell them that you found out your ex is using your property as collateral. You have EVERY right to do this without telling them how you found out.

Good luck!