I want to send an email to two people. One of them is a reader in India and the other is a postdoctoral research assistant in Germany. I do not know how to address each of them at the start of the email - “Dear Professor", "Dear Dr.” or what?
The formal way to address a postdoc in Germany would be Dr. [Surname]. My experience is that at least among Phd students/Postdocs people will quickly switch to an informal first-name address. But for first contact you should use the formal Dr. [Surname].