You may have seen the following message in your Heymarket Support Chat:
"Hi, this is Heymarket Support. An inbox, Inbox 1, has reached 75/90/100% of its included monthly messages. It can continue to send and receive messages. Please reply with questions. - Heymarket"
The above message refers to how many messages you've sent/received during your currently Heymarket billing cycle. Your Heymarket billing cycle is based on the date of your recurring charge. For example, if you are charged on the 5th of each month, in January your messaging cycle would from January 5th to February 4th.
The percentage is based on the Heymarket plan you're on. If you are currently on the Premium monthly plan, that means you get 1000 sent/received messages during your messaging cycle. You get alerts when you reach 75%, 90%, and 100% of your monthly allotment. After the 100% alert, each additional message is $0.06/message. The additional message fee will be charged on Sundays.