Fix Zohomail connection issue to my server

I need to someone to fix a "refuse server connection" from Zohomail to my Server and also explain what was wrong.

I have full Admin Access to my Server on Ionos using Plesk and SSH and of course to Zoho One as Super Admin.

I still have access to my old server but it will stop in a few days.

I have a few domains, but I'm looking to have one particular email address sending and receiving through zohomail. The other domains are not yet pointing to the new server. If I can understand and fix this one, then then I should be able to correct and point the other ones.

I am fairly conversant with the interfaces, and both Zoho and Ionos are blaming each other.

I can send and receive in webmail using Roundcube, so Ionos say it's not their problem. Zoho say it's the fault of the server.

To someone who knows what they are doing, this should be a fairly quick fix. We can surely see this fixed by using remote screen sharing, so you should be able to work with me on that front, or we can use a teams/zoom session at a mutually convenient time.

I may have other Zoho One work in the near future as well, as we are going to launch our new site soon.