'Dear Friends' versus 'Dear friends'

English as a Second Language English Vocabulary, Grammar and Idioms
1

Hi

I am often confused at the start of some formal or informal letter.

Dear Friends or
Dear friends

Dear All or
Dear all

Dear Colleagues or
Dear colleagues

Is it a metter of personal choice? Or there is some rule?

Tom

2

Hi Tom

If the letter were formal, I’d capitalize: Dear Colleagues,

Amy

3

Me too.