The SMTP 500 error can occur if the SMTP command used is not recognized or supported by the receiving SMTP server or if the SMTP command is too long.
Firstly, check the length of the SMTP command string. Ensure that the command length does not exceed the maximum length supported by the SMTP implementation.
Secondly, ensure that the receiving SMTP server supports the command you are trying to execute.
Lastly, disabling the anti-virus guard may help solve an SMTP 500 Error.
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