In this article i will show how to fix error in VestaCP:
An error occurred!
SMTP Error (550): Failed to set sender "xxx.zzz" (Access denied - Invalid HELO name (See RFC2821 4.1.3)).
The problem happens when you try to send email from Roundcube webmail.
![Roundcube SMTP Error (550)](https://i0.wp.com/arstech.net/wp-content/uploads/2020/08/Roundcube-SMTP-Error-550.png?resize=640%2C224&ssl=1)
To fix this error you need change smtp_helo_host to localhost in file defaults.inc.php. Find defaults.inc.php file in /etc/roundcubemail/ or /etc/roundcube/ directories.
And update the variable smtp_helo_host to localhost as shown on pic:
![smtp_helo_host](https://i0.wp.com/arstech.net/wp-content/uploads/2020/08/smtp_helo_host.png?resize=640%2C444&ssl=1)