I'm not a native English speaker so I wasn't exactly aware of the Mr/Ms + last name rule. I addressed the HR Director of a potential employer with Ms + her first name. My native friend then told me that it didn't make any sense.
In my culture, using the equivalent of Mr/Ms with first name is a sign of respect and doesn't have any negative cultural connotations. Is it a deal breaker to combine her first name with the title Ms? Should I apologize?
There is no need to apologise for this. This requires no more apology than the time you didn't use "the" in the right place. You are, as you said, a non-native speaker of English. We expect non-native speakers to have a non-native level of English, and if you make mistakes that is fine and expected. If I (even as a native speaker) apologised for every grammar mistake I make, I'd spend my life apologising.
I think you may be over-worried about using Ms+first name. It isn't an error, it just isn't often used (in the past it was used by servants when talking to the children of superiors, but this context doesn't exist in current society). You'll probably now be aware that "Mr" or "Ms" is rarely used when talking to a person. But commonly used, with last name when talking about a person to a person who doesn't know them.
{referring to Jane Atkins}
Hello Jane, can I ask you a question about the crossbeam
{later to a colleague} Jane said that the crossbeam has gone out of skew on the treadle.
{later to an outside company} Please send the invoice for treadle adjustment to Ms Atkins.
No need to apologize. The combination Ms./Mr./Mrs. + first name is actually not uncommon in the U.S. I had an advisor in college who asked us to address her as "Dr. Mary".
In formal settings of course this combination is likely inappropriate. Using Mr./Ms./Mrs. indicates formality and politeness, whereas addressing someone by their first name shows familiarity and casualness. Therefore only in certain settings is this combination considered appropriate and natural. Good thing we don't live in the world of Jane Austin. You don't need to worry too much about this. Although in the workplace this combination is unexpected, I doubt the person addressed would take umbrage at it. Of course how jarring title + first name sounds also depends on where in the anglophone world you are and who you are dealing with. Some places/people are more traditional than others.
You mentioned in your culture "using the equivalent of Mr/Ms with first name is a sign of respect and doesn't have any negative cultural connotations." It makes me wonder if you are Turkish or from a similar culture.
Also see this ELU post. As several answers there have mentioned, this combination occurs most likely with children addressing older people. I would like to add that in my experience this is not necessarily a Southern thing only.
You are communicating with the HR Director, then it seems you should be formal, there are some suggestions:
Here are the formal titles English speakers use :
1- Mr + last name (any man)
2- Mrs + last name (married woman who uses her husband's last name)
3- Ms + last name (married or unmarried woman; common in business)
4- Miss + last name (unmarried woman)
5- Dr. + last name (some doctors go by Dr + first name)
6- Professor (Prof.) + last name (in a university setting)
But, if you sent your email with first name, it does not need to apologize, just in the next email use the correct form.
If I were you, I just wrote "Dear (First Name) + Last Name", I always use it in my email. And, If they are Dr. or Prof., I just add Dr. or Prof. after Dear.
All forms of address such as Ms, Mr or Mrs. are used with the last name, not the first name, unless you know the person.
Reference (when you know the person's name)
1- Examples of how to address a cover letter if you know the hiring manager Dear Mr. Smith, Dear Allen, Dear Ms. Rachel Johnson, If you know who the hiring manager is, but have never met them, you should start off the letter by addressing them directly and giving a brief introduction of who you are. For context, you can explain where you found his or her name or the opportunity.
2 -
If you are certain of their gender and want to use a title, use either Mr. or Ms. Avoid using Mrs. or Miss since this will involve some guesswork about their marital status. (You may make an exception if you know the hiring manager personally and they have told you their preference.) Follow the title with their last name. For example:
Dear Ms. Greene Dear Mr. Johnson
Use Mrs. if that is what the HR person uses and you know that.
