in reply to Tuta

Situation dependent. For formal communication I adhere to the rules learnt from my early years.

To Whom It May Concern, if the identity of the recipient, within an organization, isn’t readily known.

Dear XYZ/Gentili Signori(a) XYZ, if the recipient’s identity is known.

Hi — for follow up correspondences to the Dear XYZ recipient.

For all other informal communications, I adhere to no rules and simply go with whatever comes to mind. For example: Hey, Ciao, Bonjour etc