If you receive sendmail 451 error, it is most likely due to lack of Sendgrid credit.
Sengrid credit limit gets automatically reset if all send/receive statistics are good.
For users with low mail reputation, the automatic credit reset process is stopped in order to protect Sendgrid from being blocked as SPAM.
If you are seeing that error, it can mean that you are:
- trying to send a lot of emails to a test email address,
- you are sending emails to address that don't exist, ... review your email list to make sure that the recipients exists.
To know for sure, open a support ticket, and we will recommend which action to take and can perform a manual credit limit reset for some edge cases.