The SMTP server may display the SMTP error 421 in many instances since this error can have numerous variations. However, it is most likely a temporary issue within the mail server or the recipient's email account.
The most common causes for this error are:
Since these errors are mostly temporary, the user should wait for a considerable amount of time before attempting to send an email to the same server or connect to the same server.
However, in instances when the error states that there are multiple connections from or to the host, it is advisable to adhere to any limits within the email server.
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