I have a Chinese friend/colleague who will occasionally address an email to me with “Dear Ada” (Ada is not my name). He usually does it when the email is personal & not work-related. Can anyone tell me if this is a special greeting in Chinese? Thanks!
I’ll be no help with the Chinese query, however, Are you, by chance, a programmer? and is your colleague familiar with Babbage & Co.?
In South China, people tend to address people with a sound like “ah” added to the front of their name (like ah-Jay). It is a friendly way to call people’s name or title. I have no clue what “Ada” is, unless your name starts with “D,” like “Dan.”
Is your friends name “Ada” by any chance? Or as China Girl alluded to, is your friends name somehow associated with the sound “da” like Dan for example? If that is true, then it would be more like “Ah-Dan”.
The “Ah” is used primarily in Southern China like Taiwan or Guangdong. It is used for childrens nicknames or for friends. So, back to the example of Dan, “Ah-Dan” shortened to “Ah-Da” would be like calling him “Danny”
Otherwise, I have no idea what it means.
Thanks for the suggestions, folks, but I guess this must just be some personal quirk of his - my name is Linda, we are not programmers, and I am fairly confident he hasn’t been doing much reading about difference engines lately. 
He is from South China, though, so I suppose the explanation is connected to what China Girl and China Guy have posted. (Maybe “Ah” plus “da” from “Linda”?)
This might be it. A Taiwanese friend of mine called our Japanese friend Madoka “Ah-ka”, so “Ah-da” for “Linda” seem plausible to me.