What does MBR mean when EIN is issued?

MBR stands for "Mail Box Rental." When the Internal Revenue Service (IRS) issues an Employer Identification Number (EIN) to a business, it needs a physical address to send correspondence and tax forms. If a business does not have a physical location, it can use a mail box rental address as its business address. This address will then be listed on the EIN letter that the IRS sends to the business.