You can increase your efficiency by using your e-mail program's spell checker because it eliminates the need for you to proofread your messages before sending them out.
Using the e-mails program to spell check your messages does improve your efficiency however, it's still very important to proofread your messages before sending them out. Just because you know the words are spelled correctly, does not mean that the content is correct or that the worlds are being used in the correct format. Proof reading your messages is highly incouraged to check for errors.