I am currently writing an e-mail to a member of Oxbridge staff. He is a "lecturer," full-time staff, who co-ordinates a well-known program, but does not hold a PhD, only an MA.
I can't call him "Dr. ," or even "Professor ," so is "Mr." sufficient? It seems too informal for someone in such a high-ranking position.
"Mr" should be fine (though note that British style drops the period from common honorifics). For example, Trinity College Cambridge's list of fellows has two "Mr"s and four "Ms"s.
However, be very sure that the lecturer you're emailing doesn't have a doctorate. Most Oxbridge (and I believe UK) lecturers are equivalent to some sort of professor in the US system, and the title of professor is generally reserved for academics somewhere between "regular member of teaching/research staff" and "department head". Accordingly, most UK lecturers do have a PhD.
If you're not sure, it's probably better to go with "Dr" rather than "Mr", as if you do get it wrong one way or the other, that's the one that's less likely to offend.
It is better to amuse your addressee than risk causing offence. A journal editor addressed me as "Professor" in all email correspondence, even though I (twice) told him that I was only "Mr". This amused me and caused no offence. But I can imagine that addressing a real Doctor or Professor as "Mr" would not be well taken.
While the etiquette for salutations in formal letters is pretty well established, email etiquette is less clear. While this question on the English Language SE was not particularly well received, I particularly like this answer. The key part of that answer is that because of the prevalence of spam you need to establish who you are and why you are contacting the person as quickly as possible and not waste the valuable first line with a redundant salutation. If you drop the salutation (which some would say is the proper etiquette), you avoid the issue of how to address the individual.
Mr. is fine. Don't worry it's not that big a deal just because they go to a famous university. They are still just regular people. Just be super polite in the tone of the email.
Option 2 is to avoid using the term "Mr.", and just to write simply "Hi", or "Dear Sir"
You probably have multiple aims here:
First of all, do your research: how is the person you wish to approach described on their institution's website? In extreme cases you might wish to consult a modern book of etiquette. For example, if you were addressing yourself to "Professor, the Right Honourable, the Lord X" you need to know that 'Dear Lord X' is fine and that 'My Lord' is now ridiculous.
Secondly, do not go over the top in honorific titles. (I still giggle at once being addressed as 'Your Excellency' in an unsolicited email.)
Thirdly avoid generic titles without a name, such as 'Dear Professor' or, even worse, 'Dear Doctor'. 'Dear Sir' might once have been OK, but these days it feels as if the writer has not done their research.
All of the above will probably permit you to avoid offending the recipient. But remember that, in the UK at least, a doctorate is not necessarily a requirement for a senior academic position. I have known very renowned, actually world famous, academics who never proceeded beyond an undergraduate degree and also some whose published books established a reputation before they had the chance to finish a formal doctorate so they never bothered. Some of those might have been mildly offended at being addressed as 'Doctor.'
All of this points to finding out something about the person to whom you wish to write before actually doing so.
