If you’d like to come across as friendly and upbeat in an email, change the font...

There’s an effective trick you can try in your emails that can change the tone. Increase the font size. The trick is, just do that on key words. By slightly increasing the font on positive words, such as benefit, thank you or appreciate, the overall tone of your email will improve. People will notice that slightly larger font more, so use it wisely. 

Avoid using whimsical fonts, even if you’re trying to lighten things up. Those just distract from what you’re trying to say and tend to look too casual if it’s a business email. Stick to Arial and Times Roman for those.