'Dear Friends' versus 'Dear friends'

[Tom]

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

[Yankee]

Hi Tom

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

Amy
_________________
Amy
.
ESL teacher, translator, and a native speaker of American English

[Pamela]

Me too.