Horse 2380 - Always Dr.; Never Mr.

In my working career, I have worked in many places but the two most notable were the Commonwealth Law Courts and the accounting firm where I currently work. As you'd expect in these environments, I have met a lot of hurting, angry, belligerent, and downright nasty people. Due to the very nature of both money and the law, which act as conduits and proxies for the exercise of power, money and the law tends to attract a higher than normal ratio of narcissists, sociopaths, and people for whom superbia is not a deadly sin but has been twisted into a barbed weapon which is glorified.
In such an environment, you very quickly learn that everything in print can and will be used against you in the court of knavery, where the narcissists, sociopaths and superbians act as judge, jury, executioner and torturer because they find sport and fun in it. For someone like me who likes to use a thousand words to paint a more accurate picture, this is an invitation to commit career suicide on an almost daily basis; so my emails and correspondence tends to be as economical as possible, sparse and terse. You also very quickly learn to address people with calm respect in case they decide to bite your head off and you also learn never to use any gender specific titles or pronouns for the same reason.

Mr, Mrs, Ms, are all out. The risk of mistaking a Jo, Sam, Djomba, Kim, or Morgan, for any gender, is like walking around with a hand grenade with the pin taken out. It might very well be a dud and nothing will happen but it might also be quietly ticking away and ready to explode in your face at an unknown moment. Every envelope is addressed "Name Surname" and "Dear Name," and even then you still need to be careful.
I'm also extremely wary about whose name is on an envelope if I have to send multiple documents to a couple, be they married or otherwise. You can always presume that any missteps will be punished. My general principle is that if I have received a letter and documents in the mail, that the return mail is addressed to them and where you have two surnames, two pieces of mail will be sent back.

There is of course one absolutely major exception and to miss that is also to invite someone to bite your head off; that is the use of formal titles. In cases like Doctor and Professor, these are the result of many years of work or where a university has conferred the honour on someone. I always without exception include the titles of Doctor or Professor, in spite of the fact that I recently received an objection where someone accuse me of "sexism heteronormative"; which made no sense to me, considering that it was on their business card which was stapled to the letter they'd sent us in the first place.
Military ranks are even more fraught with danger and you should always include those; that also goes for "Sir" and "Lady", where the title has been acknowledged by royalty. I only have to send out one letter of those types in a year and although "Lady Name" will try to play you with tea and biscuits and will talk the hind leg off a dog, she is one of the most gracious and lovely people in the world. "Sir Name" is a superb narcissistic sociopath (please see above) and should be passed as one rounds a navigation buoy.

The other thing that I'm really afraid of is the opening and salutation of any piece of correspondence. The opening of "Dear Name" is acceptable but even then i have still had one person complain that I wasn't dear to them and that they weren't dear to me, and so with that client specifically I will open a letter with "Name," which seems to have calmed them.
Emails are also maddening and I've long since decided that "Good Morning," "Good Afternoon," and "Good Evening," are acceptable openings but for reasons which are insensible to me, those kind of openings on printed correspondence just look plain wrong.
Every letter, every email, irrespective of whether or not I barely know them or am friends with them or even my wife, will always end "Thank you,". Again, I have been bitten in the past and so I dropped any notion of using "Yours sincerely," when nobody is ever sincere about anything and the word "Yours" might imply a level of closeness which given my mistrust of humanity, is almost never allowed.

The two major exceptions to all of this is correspondence which I send to a French lady in her 90's who I'm convinced was a spy, or in the Communist Resistance, or perhaps some kind of industrial espionage agent, who is always addressed as "Mme," and an ex-Army officer where the opening is always "Dear Major," and never their first name and the salutation is always "Memento Mori," which to be honest should be the correct sign off to every piece of correspondence ever, because the grave is always open and Death will keep its appointment with everyone; no correspondence entered into.