Make sure you make the mailings by certified mail with a return receipt request. Keep copies of what you sent. If the envelop comes back unopened, do not open. It is always possible that you may need to appear in court and present the attempted mailing to a Judge to open and read. Get an address from your tenants. If they refuse or otherwise fail to provide you with an address, mail it to your rental which would be their last known address. It is OK that it is returned to you. This will be evidence that you attempted to meet your legal requirement to get the notice out. You may also assume that their mail will be forwarded. Either way, you will have met your requirement.