I guess my first instinct might be thinking it’s rude but having seen it a few times I do get it. It really makes sense for people who get dozens or hundreds of emails a day and are gone for extended periods of time. It’s more courteous to be up front about it than silently delete like some people said they do (though I get they maybe just didn’t realize they’d need to do that).
The key to a good note is to let the person feel that you are really thinking of them. A good note keys into the person it's being sent to and reflects a special connection. (If you're writing to someone you don't know well, this is even harder. But I'll get to that later.) 1. Don't write the note when you haven't got a lot of time to think and ...

A. International House’s front desk will be open for students 24 hours each day throughout winter break, except on the holidays – Dec. 25 and Jan. 1. However, there will be professional staff on call 24 hours a day throughout the break to adequately serve students. The Front Desk may be contacted at 419.530.1600; this phone will be answered 24/7, even when the desk is closed on both holidays.
Given free rein, I’d absolutely love to tell people that needing me to show them how to do X in Excel is actually not a vacation-interrupting emergency and there are tons of free videos that would explain that, if they did not want to contact the actual departments who handle tech support and training. Or that this project they’ve known about for a month but decided to keep under their hat until it became an emergency is something they’ll need to resolve themselves. But that would not fly at all.