It's really quite simple. People have a name, they're generally quite attached to it, and they're often very attached to the way they spell it. So be polite, spell their name correctly.
Now, if you don't know how they spell their name, it's okay to get it wrong. Once. And they should not be offended, they have no good claim to be offended. They should simply say, "Sorry, that's not how I spell my name, I spell it ...."
And then you should take notice of this and from then on be a proficient and diligent speller of their name.
Particularly bad is when their name is right in front of you. Like when they've sent you an email and their name is in the address field, and they've signed it, and you're replying back to them, and you just ignore what they wrote and use whatever spelling seems good to you. Now you're just be rude, probably through mental indolence.
I'm not particularly offended when you spell my name wrong, just a little baffled that you didn't take more care, especially when it was right in front of you all along.
Seumas. It's Scottish, not Irish. That's why you don't spell it Seamus.
Macdonald, not McDonald. Again, I'm not Irish, and we don't run a family restaurant.
Thank you. Here ends today's public service announcement.
p.s. Substituting random Scottish names, a la Angus, Fergus, etc., only secures you a place in my hall of infamy
p.p.s. While I appreciate the genetic memory that drudges up Hamish as the Gaelic vocative form of Seumas, it is also generally not to be used for my name, unless you are in fact addressing me in Gaelic.