An inquiry letter, also known as a prospecting letter or letter of interest, is sent to companies that may be hiring, but, haven't advertised job openings.
Inquiry letters can be send via mail or email. However, sometimes a mailed written letter can make more of an impression than an email message which may not be opened.
Inquiry letters should contain information on why the company interests you and why your skills and experience would be an asset to the company. Also provide information on how you will follow-up and your contact information.
example:
