Styling can be difficult to learn, since, well, you can't read a person's body language when they talk over text. If you are learning English for business reasons though, the styling choices you make may affect the way emails are perceived; if the reader is aware that English is not your native language though, it should be less of a problem.
1. CAPS LOCK
Typing a sentence in all capitals gives the impression of SCREAMING (or YELLING) at the person, or really, really great added emphasis. You are either RAISING YOUR VOICE, or saying something with GREAT added emphasis. But not just emphasis, it's very heavy. IF YOU TYPE AN ENTIRE SENTENCE IN CAPS LOCK, HOWEVER, YOU MIGHT AS WELL SCREAM AT THE PERSON. It can be a threatening gesture, so be careful with it; usually it's threatening or used to display anger of some sort.
This is rarely ever used in business email, with the exceptions of 1) older folk that didn't grow up using computers, and 2) your boss is very, very upset with you. Usually in that case though, they'll just call you into their office...
2. Bold text.
Bold text calls attention to something and makes it big; it can be used for the beginning of a section in some informational pamphlet. When used in the middle of a sentence, you call a great deal of emphasis to it. Usually, you use it for important dates, locations, and times. For example, here is an appropriate usage of bold font:
"Your reservation will be at the Hilton Hotel, Los Angeles for 8/1-8/5; check in time is 3:00 p.m. and after."
So key information, essentially. This makes it really helpful for someone who wants to skip the rest of the email, and just wants to know where/when they need to be.
If your email has multiple sections, you can use boldface, possibly with underlines, for the titles of each section.
Boldface itself isn't threatening or negative, but in some contexts can make you sound pompous or snobby. It could give the impression that you are assuming that the reader doesn't know what is boldfaced, which could be taken the wrong way with some people. For example, if you told your boss:
Please don't forget to put the letter in the envelope. <--This one gives the impression of "oh, you're so stupid you can't put a letter in the envelope," or "you've messed up in the past so it's in bold this time so your idiot self doesn't forget it this time"
Please don't forget to put the letter in the envelope. <--Ehhhh....better than the one above, but I'd probably say something that implies the letter needs to be in the envelope itself. So perhaps:
I'll pick up the envelope with the letter in it when I get back. <--This is somewhat indirect, and is less likely to offend a sensitive boss. Americans love to assume.
