roundcube SMTP error (450) Recipient address rejected: Access denied

 Log in to roundcube and use my domain name mailbox to send mail, the following error message appears:
SMTP error (450) Recipient address rejected: Access denied


I don't know what went wrong? I speculate that it may be a problem with roundcube's MX records.

I use an external DNS (non-CWP DNS). How do I set up roundcube's MX record?

Maybe your localhost/server is not allowed to send email.

Check:
/etc/postfix/main.cf
and add permit_mynetworks to smtpd_recipient_restrictions example:

smtpd_recipient_restrictions =
    permit_sasl_authenticated,
    permit_mynetworks,
    reject_unauth_destination,
    reject_non_fqdn_sender,
    reject_non_fqdn_recipient,
    reject_unknown_recipient_domain,
    check_sender_access hash:/etc/postfix/sender_access
    ,reject_rbl_client zen.spamhaus.org
    ,reject_rbl_client bl.spamcop.net


and add to /etc/postfix/mynetworks file
127.0.0.0/8

Reload postfix with:
systemtl reload postfix.service