Is illegal to take pictures of another person's unopened mail?

I know it is illegal to put anything that is not stamped in a mail box but I also would like to know if it is illegal to open a mailbox for example you are lost and need an address would it be illegal to open a mailbox to see what the address is on an envelope?

Yes, it is illegal. It is a federal offeense to open a mailbox or tamper with a mail box unless you are employed by the USPS.