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.
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: