How to “properly address” attorneys

I got an email from our referral attorney today that began “Hey (First Name),” and I used his first name in my reply. My boss pulled me into his office and told me whenever staff is speaking to an attorney it should be Mr./Ms LastName. He’s corrected me on this once before and I’ve been really good about properly addressing the defense attorneys, but I slipped up thinking that because this is referral counsel and he’s so chill and casual with me that it would be fine. I apologized and told him I’ll never do it again. I mostly say good morning/ good afternoon anyway. It’s also worth noting that at our firm we all call each other by first names including the partners. In some ways Im bothered by the fact that he thinks it’s important for me to show respect to attorneys by calling them Mr. LastName after they’ve used my first name. The implication there is that I don’t deserve the same respect as them because I am not an attorney, right? What do y’all think?