1. Go to https://mxtoolbox.com/MXLookup.aspx - write the domain you want to check - and press enter:
2. Click on IP shown on the previous screenshot, it would lead us to Reverse DNS lookup (PTR), so on this page we can see what the hostname of that server is:
Here we can see that emails for this domain are hosted on the server hostname highlighted in the picture above.
If it is your server, here is how to find on what account the domain is located: viewtopic.php?f=17&t=603