EHLO incorrect (short / invalid) is not accepted

Is your hostname a fully qualified domain name such as server.mydomain.com on the machine?

You can do this command to find your server's hostname in root SSH:

Code:

hostname

If you only show server but not server.mydomain.com as the return, then your hostname isn't a fully qualified domain name and you would want to change it in WHM > Change Hostname area.

Next, does the hostname match the IP you have for it on the machine? You can run this command to see the main server IP that goes with that hostname:

Code:

hostname -i

You can then find the PTR (reverse DNS) for that IP:

Code:

dig -x IP# +short

Replace IP# with the actual IP number for your hostname. If the IP's PTR and the hostname name do not match, the provider for that IP would be able to set the PTR record for you if you contact that provider to request it.