Do you write:
A) Dear John
B) Dear Smith
C) Dear John Smith
D) Dear Mr Smith
Each of the above ways of addressing this HR manager conveys a very different attitude.
(A) gives off a feeling of chumminess. As if this guy is your friend. You may do this if you actually know the guy in person and go out drinking with him on a regular basis. If you don't know him though, it just sounds weird and disrespectful. Using this is a good way to get your resume/CV thrown in the trash can.
(B) is problematic. When you refer to someone solely by their surname in English, it can be seen as a sign of disrespect. Usually its okay in informal situations, where its easier just to refer to someone(usually famous or infamous) by their last name, however when we apply it to more formal situations, such as job applications, business letters, news, or debates, then it is almost universally seen as disrespectful. It's almost passive-aggressive in tone and gives off the impression that you have a massive axe to grind. Use this form on your cover letter and you might as well just trash your resume/CV right there and now.
(C) is better, but its not really that formal. You're not showing respect, but at the same time you are not showing any disrespect. It's pretty neutral.
(D) involves the use of a title (Mr.) It's formal, polite and deferential towards the person you are writing to. You are showing them respect, basic etiquette for person to person interactions in business, politics and professional life.
Why do I bring up politics here? Usually when politicians are debating each other, they have a tendency to use deferential titles when referring to their opponents. In the UK(and other commonwealth countries if I recall correctly) for example, its common to use the term "The right honourable" when MPs or MSPs debate with each other. It's a polite way of showing respect to the station that the MP/MSP occupies, even if you are about bulldoze every policy they stand for in your speech. Outside of the bubbles that are Westminister and Holyrood, its more common to just use the term Mr./Mrs./Ms. to show respect. If you want to speak more informally(or if you don't like the person), you may just say their full name, or if you know your opponent well you may just even say the first name.
Using the last name to refer to an opponent is usually taboo. You just don't do it in politics unless you really have it out for your opponent. Again, in general discourse there is usually nothing wrong with it, but when it comes to debates, political rallies and political interviews. It comes across as passive-aggressive. It's not often you hear politicians refer to other politicians by the last name, and when it does happen, its almost always because they have a strong dislike for that person. Margaret Thatcher is one of those people who was routinely subject to that by her opponents. George W Bush and Barack Obama are some American politicians who are/were often subject to the same passive aggressive disrespect by their political opponents.
Back in 2010, during the UK general elections that year, the leaders of the main parties had televised debates for the first time ever. As they were debating they had to think of a way to refer to each other. The opposition party leaders hated the incumbent prime minister, Gordon Brown. Everyone knew that. They didn't want to be deferential to him so they opted not to refer to him as Mr. Brown. They also didn't want to appear too friendly, so opted not to use Gordon. They also opted not to use "Brown" because it comes across as rude and alienating, so they decided to just refer to him by his full name, Gordon Brown. It does the job without seeming too deferential or rude. They came out of the debate looking quite good.
Much the same can't quite be said of Mr. Alistair Darling in the video below.
He refers to the man he sees as his political opponent by his surname throughout the interview. A big No-no move that makes him come off as a hack, rather than a former chancellor of the United Kingdom of Great Britain and Northern Ireland. In politics, you can get away with not showing respect by just using the full name rather than a title, but when you verge into disrespect, it does raise eyebrows about your conduct. It's pretty cringeworthy and you'd be unlikely to see a more able politician, such as the Prime Minister delve into such pettiness. Mr Cameron did manage to avoid that 4 years ago, despite his clear hatred for his foe, Gordon Brown. As the head of Westministers Better Together cohort, Mr. Darling should know better. However his actions indicate those of one who has risen too far above his ability.