It is vital that you make your cover letter perfect. A hiring manager may often receive 100 applicants per position and yet they have to cut 95 of them so that they have 5 top candidates to interview.
As you might imagine, with so many to cut out it doesnt take much for the recruiter to decide to throw away a resume that they received. Any poorly formatted, long-winded, or boring cover letter will simply be deleted.
To make sure your cover letter wins you the interview you should consider getting resume professionals like Resume Scorecard to help.