I usually let this go (since it doesn't happen often) but called someone out on an email exchange between us where my name went from 'Danny' the first 5 or so times to 'Dani' the last two.
Clearly knew my name, since it's everywhere on the emails, so it's either laziness or intent.
I freaking HATE this. My name is say... Tom. My email is Tom, signature is Tom. Everything says Tom, then some fucker comes along and goes "Thomas". Bro, it aint my name, your just trying to power trip.
Lol they're trying to get under your skin. Don't even mention it. If they cannot get your name right (which they obviously can), then playing into their hand is only going to enable them further and have them keep pushing your buttons.
Let it be water off your back, and know that you would never stoop as low as them.
My last name can be a first name and it really bothers me when people address me by my last name. It shows you're not paying attention to anything if you're not looking at the sent name or my sign off