i need help please!!

OS Ubuntu 20.04
LAMP comp.
osTicket 1.15.1

i have error in smtp: [SMTP: Invalid response code received from server (code: 550, response: Access denied - Invalid HELO name (See RFC2821 4.1.1.1))]

ubuntu server 20.04 in VPS
LAMP apache2, php7.4, mysql mariadb

What else I need to install?