Which ones are correct:
'Hi All' or is it 'Hi all'. (A)ll does not look right
'Hi Peter/John' or 'Hi Peter, John' when you want to specifically address the e-mail to two people, i.e. so they clearly know they are the main addresseess and appear in 'to' and all the rest, if any, are in the 'cc'.
Best Answer
There's no need to capitalise because 'all' is a common noun.
In the UK it's still more common to use a '/' or 'and' than a serial comma when addressing more than one person in an email. But then names are sometimes omitted entirely.