I have been dealing with the SENCO at DS' school for years. From the start, I was a little surprised that when I emailed him the first time, he replied calling me by my first name as obviously, the teachers are all Mr/Mrs/Ms etc. But I chalked it up to the fact that I'd emailed him first and of course, had not signed off as "MsIsBest" and I let it go.
DH has a very minor relationship with this man through his work - the man is a customer of DH's company and has seen/spoken to DH there a few times. First names all round.
DH does not usually get directly involved but has been attending meetings. From memory, I'm pretty sure the SENCO called us both by our first names in those meetings and he definitely did in a joint email to us. ie when it's him AND me, it's first names.
Today, DH emailed him. He has replied, "Dear Mr DHSurname".
It's a minor thing but I am incandescent. I can't talk about it in real life because people will tell me that I am over reacting etc etc. But it's these small things that annoy me so much.