When emailing at work to the in-house team I tend to start with Hello {first name} because it sounds open and friendly .
Rarely a Dear {first name} if it's something formal like performance management type things or I'm being passive aggressive
To external people its a Dear until I get to know them and then it seems too formal
The one thing that does set my teeth on edge is when people begin their emails with just the persons first name as in...
Doris,
Further to our meeting this morning,,,
AIBU to think that appears brusque or rude?