It is the e-mail rate accepted by the ISPs, such message service providers as Hotmail, Yahoo, Gmail.

For example, even if you send a total of 10,000 e-mails per hour, not all of these messages may be delivered to recipients. The Message Service Providers (ISP) decide how many e-mails will be delivered to the recipients. The percentage of the e-mails delivered among those 10,000 e-mails you have sent is the acceptance rate.