Five Sins of Email
Shaykh Faraz Rabbani (Allah preserve him) said,

“(1) Not being clear
Go out of your way to explain yourself, clearly, especially when correcting others or expressing disagreement.
(2) Going on and on
People are busy. Respect that, and get to your point quickly and clearly.
(3) Sending huge attachments
People use email for work. Huge attachments slow them down.
(4) Writing poorly or being too curt
This turns people off what you write; and can lead to misunderstandings or (worse) arguments.
(5) Not using clear subject lines
People are busy. Tell them what you’re writing about in the subject line. Saying, ‘Important,’ or ‘Read this,’ is not helpful.”

Source: Seeker’s Digest