Now, co-workers typically use email to convey information to each other, and to non-employees outside the office. Memos are still used in situations where a message is meant to accompany a specific file and in cases that require more privacy than an email. Much like Business Letters, a company's email uniformity is a must.
Commercium eius epistularum curabam: Many people who send business correspondence by email do not realize there is a big difference between using it for that purpose and for communicating with friends and family. When you send a message to a buddy or relative, you can be pretty casual. You can use slang if you want or may even spice it up with inappropriate language if you know the recipient won't have a problem with that. In other words, you can write as if you were speaking to that person.
Business correspondence is quite different. Good email etiquette, actually business etiquette in general, dictates that you conduct yourself with professionalism. When you are too casual, use slang or inappropriate language, or send error-filled messages, it leaves a bad impression on the recipients of your message.