If you receive the following error when sending e-mail via Plesk Roundcube, the problem will be solved if you update it to smtpd_use_tls=no via Postfix settings.
SMTP authentication fails with SMTP Error (250): Authentication failed
# postconf -e smtpd_tls_security_level=may
# postconf -e smtpd_use_tls=no
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