You may run into the error if the anti-spam filter, anti-virus guard, or firewall prevents the message from sending. In most cases, your anti-virus guard may prevent the email from sending, or your email may contain spam-like phrases that get flagged as spam by your anti-spam filter.
Firstly, verify that your message does not contain any possible spam words. For example, do not include phrases such as "You have won" or "Claim your reward" or language that may sound spam.
Secondly, disable your anti-virus guard/ firewall or anti-spam filter and try sending the email again.
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