Your cover letter is the first thing a recruiter will see, so it is essential that you introduce your CV in the best way possible. Although there's no such thing as the perfect letter, following a basic formula will help make your cover letter a worthy accompaniment to your CV.
The key to making an impact when you are emailing an application is to customise it as much as possible to the job for which you are applying.
There are a three things to remember when you apply by email: