While writing an "effective" email, it is important that the sender considers the way the reader will perceive the note.
For your convenience, the Huffington Post has compiled a list of do's and don'ts for the most productive mail.
The things to keep in mind in do's is to keep the mail brief, straight foreword and limiting it to one topic only, while being courteous.
On the other hand, don't write anything private or confidential, or forget to proofread.
Also, if an issue can be resolved through a simple phone call then skip the formality of writing an email.