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To add more details, this is usually caused by attempting to send a message with a gmail.com domain From address using a sending email server which is not the normal ones used by Gmail. In other words, if you want your From address to be at the gmail.com domain, you need to use Google SMTP servers to send the message, which usually means you need to send the message directly from Gmail and not from some other email service or client. Look up SPF, DKIM, and DMARC at Wikipedia to see how many companies are preventing spam.
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