how can I solve this problem: Diagnostic-Code: smtp; 501 HELO requires FQDN address

[cigikey]

Some email addresses sebd back the messages with "Diagnostic-Code: smtp; 501 HELO requires FQDN address".
How can I solve this problem? If you can send a detailed description: which file(s), which line! Thanks!

[alec]

You need to set properly smtp_helo_host option in config/main.inc.php file.

[cigikey]

You need to set properly smtp_helo_host option in config/main.inc.php file.

OK! I found: /etc/roundcube/main.inc.php; var/lib/roundcube/config/main.inc.php.
Which one should be edit? Both files in the smtp_helo_host_option section are this: rcmail_config['smtp_helo_host']=  '' ;
What and what needs to be replaced? Thanks.