My concern is that if I send my application to an employer through
email, it is likely that he/she would not be able to read it
thoroughly, given the number of application emails he/ she receives.
But would sending my application through a courier hurt my chances of
landing the job?
You would look foolish.
And you probably wouldn't increase the chances that your resume would be read any differently than any other resume.
Just because a courier delivers a resume, that doesn't mean it gets read any differently. Almost certainly it would have to go through HR first (to get logged and initially screened). And then it would just be in the same pile as all the others.
As @ChristopherEstep points out, HR may well not even know that it was couriered. The courier delivers it to reception or receiving and then it just gets put in the same pile as the rest of the mail for HR.
The only exception would be those "creative" jobs where it might be an advantage to have your resume "make an entrance" via courier, pajama-gram, or carrier pigeon. But those are few and far between.