You should first check if your office culture requires an email salutation. Many office cultures do not. If you do need a salutation, then it is better to give a clear indication of who the email applies to. For example, if everyone you are mass-mailing is in the same department or position, you could say, "Dear Marketing Department" or "Dear Marketing Management". If you cannot clearly indicate who the mail is intended for, then the default is "Dear All" or "Dear Colleagues." This is only a formality and some office cultures prefer a more casual salutation. But if you don't know, just do it and then see how your other office workers reply. If their salutation is different, copy it. If they don't have a salutation, you do not need to include one next time.

I‘d recommend „Dear Collegues“ myself. That sounds best. „Dear All“ just sounds weird to me and I’d never say that, and I’ve never heard it.