Our borrower passed away recenlty and his home which he used as collateral is in a flood hazard area. His current flood policy will expire soon so we'll need to send out the initial 45-day notice. Where should we send the letter?
We have the name/address of the attorney who is handling the estate but it has not gone through probate yet.

Send it to the executor of the estate.

Thank you for replying but I have been told that an executor has not been appointed yet.
Could I send it to "Executor for the Estate of John Doe" and mail it to the attorney's office?

That's what I would do. I would also send it certified with a return receipt requested.