You have likely been on the sending or receiving end of an email that de-motivated you or ruined someone's day. We call them e-bombs. Here's a way to avoid having such a negative effect. When you receive an e-bomb, you'll notice one consistency - the tone in which you read it. Your emphasis on words and pronunciation goes negative, even if the person sending you the email NEVER speaks that way.
Next time you are writing an email that you feel could be the least bit negative, do yourself a favor and read it back to yourself in a snippy, ugly tone. Of course, if you're writing a flowery and 100% complimentary email, it isn't an issue. If, on the other hand, you are providing a suggestion, it might sound condescending. The point is not to censor yourself, but merely to pay attention to how you word your emails so you share information with the proper tone.