Our MailChannels system is adaptive and there is no pre-set limit to how many emails you can send in a given window of time. However, as a general rule of thumb, it's good to slowly ramp the rate at which you send email in order to avoid email blocking.
We use STF block to prevent the sudden increase in sending rate from an email account, because the sudden increase of sending rate may indicate your account has been compromised.
In order to get rid of this error, you would need to slowly ramp (increase) the rate at which you send out email e.g., please maintain the sending speed by sending emails in a slow rate and then gradually increase, so this behaviour of sending will train the system that you used to send mail quickly. As the system notices that you are sending good emails that recipients appear to be accepting, the number of emails that you can send will gradually increase. For example, If you send out 300 emails in first one hour, wait for them to get delivered out, then send the emails with the count of 400 in the next hour, wait to get them delivered and then send 500 emails in next hour and so on. This will be the actual trick to send bulk emails.
Please note that sending out an email blast is a very common technique practiced by spammers to send a large volume of emails in a short period of time. Hence, we do not allow a large volume. One of the tell-tale signs of a spam attack is a sudden change in email volume from an individual sender. To prevent spam attacks before they cause damage, the MailChannels system rate limits senders who suddenly increase their email sending volume. Also note that we don't have exact preset sending limit as our system is adaptive and automatic. The best way to solve this issue is to sending the mails with small batches.