Email to two academics of differing familiarity
&quot;Dear all&quot; sounds perfectly fine to me, and using plural is an easy way out, especially if not all the recipients have a honorific. &quot;Dear professors/ colleagues/ both&quot; are other options, although in that case one of the recipients would not like the first one. For two recipients only, using both names as @vonbrand suggests is fine but I would avoid it for more than two.