We were not able to send mails from Round Cube and while trying we were getting this error.
SMTP Error (535): Authentication failed
To fix this we need to make a change in the RoundCube config file.
In a normal server , roundcube config file will be this /usr/share/roundcube/config/main.inc.php
To fix this we need to change the below value and save the config file.
$rcmail_config[‘smtp_user’] = ‘%u’;
$rcmail_config[‘smtp_user’] = ”;
After this you should be able to send mails without any issues.