SendGrid credits denote the number of emails that you can send. SendGrid uses one credit per email, and these credits get renewed at the start of every month. Therefore, you will run into this error if you exceed the maximum number of emails your SendGrid account can send.
For example, the free account lets you send 100 emails per day, and if you exceed this quota, SendGrid will throw the error - "Maximum credits exceeded."
You will have to wait until your quota gets renewed to resolve this error. If this is not an option for you, you can upgrade your SendGrid plan and get more credits based on your requirements.
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