An acceleration or demand letter is sent by the lender once the borrower has defaulted on the mortgage payments.  You have received this letter because the lender is required to send it before the lender can proceed with the foreclosure process.  The letter serves as the lender’s final attempt to collect the past due payments before the lender commences legal action to collect the debt.  Generally, you have 30 days to respond to an acceleration or demand letter if you chose to do so.