SMTP error 540 occurs because the recipient mail server rejects the emails from the sender. However, the reasons for the rejection may differ depending on the circumstance. Mostly the emails get rejected due to the recipient's inactive email account or if the recipient blocks the sender's email address, domain, or IP address.
There may be different solutions depending on the cause of the error. For example, if the recipient's email account is inactive, this may need to be brought to the attention of the recipient's email server/administrator.
Suppose the recipient's email server blocks the sender's email. In that case, this may need to be brought to the attention of the email administrator to check if any of the email security controls has blocked the sender's email address or domain.
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