The error "511 sorry, no mailbox here by that name (#5.1.1 – chkuser)" indicates that the mail server could not deliver the message because it could not find a valid mailbox with the specified name.
This could happen for several reasons:
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The email address you're trying to send the message to doesn't exist. It could be a typo or an address that is no longer in use.
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The recipient's domain is not correctly configured. There may be a problem with the mail server configuration or the domain's DNS record.
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The recipient's mail server has active protection measures, such as "chkuser" (user verification). This measure seeks to validate the existence of the mailbox before accepting the message. If the mailbox doesn't exist, the error is generated.
The last time one of our clients reported this error, they had typed gamail.com instead of gmail.com, so checking that the email is correctly spelled is usually the solution to the problem in 99% of cases.