I’d consider just using her first name. I very much doubt she’ll be all offended that you used her first name, people generally aren’t like that.
I know a number of people who might be offended by that – including me, under the right circumstances.
Thanks, this was particularly help in elucidating what I did not quite understand.
I was going to propose “S’up Boo?” as a contemporary alternative. Or you can always go with the Donald Duck classic, “Hi Toots!”
Unless I know to use something else in particular, I normally default to “Sir” or “Ma’am” when addressing people in correspondence. Some folks don’t like it, for a variety of reasons (I was once informed by a PFC in the Marine Corps that I needed to stop calling an Army Staff Sergeant “Sir” because it was blatantly disrespectful to him… he himself never said anything or seemed to take offense.
Of course, I’ve been told by folks in the other branches that the Air Force has some very strange ways of dealing between the ranks.
But yeah, back to my original point, I start with “Sir” or “Ma’am” and assume they’ll correct me if I need to switch to something else.
Sergeants/Sir - from this thread: Is a Sergeant Ever a “Sir”?
“Don’t call me sir, I work for a living!” 
The title Miz had always been used down South for women regardless of marital status. All they did was dress it up in a snappy new professional/business-oriented spelling, which freed it from associations with Granny of the Beverly Hillbillies, who had already made it familiar several years before Ms. magazine was founded in 1971. Somehow the new spelling made all the difference for the word’s image.
I use “Madam” sometimes. The apostrophe in “Ma’am” is an awkward substitute for the “d.”
Of course, like “Mistress,” this can have unfortunate (or simply inaccurate) connotations as well.
“I’m not a madam! I’m a concierge!” - from The Producers
I work with one lady who was horribly offended by “Ms.” and demanded to be addressed as “Mrs.” because “I’m married!” (the “dammit” was implied in how angry she got). She had issues I’m sure.
“Ma’am” will be disliked in some cultural circles…like mine, for example. Addressing a letter as “Dear sir or madam” is fine, but around here (Montreal) “ma’am” is pretty much only used in conversation in a condescending or sarcastic manner, and would strike most people around here as a rather bizarre thing to write. We’d probably assume you are from Texas, or a jerk. Or both.
I’d address the CEO of a company as Ms (or Mme depending on which language I spoke to her in and was writing in!) It’s the most neutral form, and is the one I also prefer. I think someone would have to seek to take offense at “Ms” while there are legitimate historical and societal reasons for disliking the connotations brought by “Miss”, “Mrs” or “Ma’am”. Besides, anyone who didn’t like how you addressed them should politely correct you (“In the future, kindly use “xx” as it’s my preference”). They can take offense when you ignore them on it!
Use “Dr” and give her a promotion. Unless she does happen to have an PhD, MD, JD, or other doctorate degree, in which case, you’re not insulting her.
Should you not address the e-mail to “Dear <firstname>”? This is a person whom you have met in a social setting, who (I presume) introduced herself by her first name and invited you to send her an e-mail. If you addressed her by her first name in conversation I would consider it a bit odd and alienating when sending her an e-mail to revert to “Dear <title> <surname>”.
You know, I’m not even MARRIED YET and we already received an invitation to a party addressed to Mr. and Mrs. Hisfirstname Hislastname?
Personally, because you met her socially I would find it appropriate to address it “Dear Firstname”; otherwise I would certainly go with “Ms.”
Corporate-type checking in here: Assuming you are in Canada or the US, I second what hibernicus and Zsofia suggested. The standard in business is that everyone is on a first-name basis. The only time people are referred to by Mr./Mrs./Ms. is if they are being addressed by those in the service industry or some old-school secretaries*.
This is assuming you’re at least a high school graduate (you say she’s twice your age; but you don’t say how old you are). If you’re still in high school, then go with Ms.
*If they’re “administrative assistants” then by definition they’re not old school enough to use Mr./Mrs.Ms.
ETA: And just be thankful you’re not French, so you don’t have to agonize over the “tu” and “vous” thing as well
I hate the ‘Ms.’ thing, BUT…it’s the only equivalent of ‘Mr.’, so, we’re stuck with it.
I’d prefer this all around! And if an email is coming to a specific position, someone doing a particular job but-who-knows-who-is-doing-it-that-day, then just a position title.
The best suggestion I’ve ever heard is to address everybody as “Ps.”
Abbreviation of “Person”
Pronounced: “Pizz.”
This was my first thought too… if you’d bumped into her and happened to recognise her, but without any personal connection, then a “Dear Ms So-and-So” would work.
But if you’ve spoken with her on a first name basis then I would follow up that introduction with a “Dear Muriel” email.
I dunno, maybe it’s just cause of most of my official email communication being in the military, but we’re discouraged from using first names of people we’re not social/authoritative equals to in emails, if only because other people may see those emails if they get forwarded along. But then, the military is a b it more obsessed with outward appearances than other social settings are, and I have no experience whatsoever with professional communications in the private market.
a) nobody cares. Use whatever in the first email, then what she signs off with in subsequent emails.
b) unless they’re super tech unsavvy, everyone reads personal correspondences. To the extent that people have assistants, it’s to filter out routine emails and cold calling. She might then fob you off to the assistant for further follow up, to schedule a meeting or to pass on certain info. That’s common.
c) to maximize the chance of something actionable, address by first name to pass through the assistant and keep it short and to the point.