Outgoing email problem -> 550 Access denied – Invalid HELO name
550 Access denied – Invalid HELO name (See RFC5321 18.104.22.168) is a very common problem when sending emails from webmail (Roundcube, Horde etc) or via SMTP.
The solution is very simple.Simply open /etc/exim.conf and edit the following line:
where at your_hostname you will add your server’s hostname. Save and restart Exim and the problem should be fixed.
Also make sure to have correct DKIM, SPF and rDNS records since they might cause this problem also.