SMTP error 521 usually occurs within mail relays, meaning that these mail servers do not accept and deliver the emails themselves but only relay them to a remote mail server. It is essential to remember that from this point onward, the emails may fail, or the email server may relay the email to another.
Before fixing this error code, it is important to check the delivery of the emails sent, as the delivery may be successful via the email relay.
Suppose the recipients confirm that the emails are not reaching them. In that case, the users need to determine why the recipient's mail server is not accepting emails by contacting the mail server administrators.
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