When adding a new mail domain to my Mailgun account, I noticed a message informing me I should use a subdomain instead. Intrigued by this I clicked on the FAQ link they provided, but unfortunately it contained little to no technical information as to why.
After some back and forth with a colleague about this we concluded that it can only hurt the reputation of Mailgun as they are "spoofing mail" from the top level domain. When we tried to add the same domain to another SMTP provider we saw the same message appear, and again, no technical information in their FAQ.
I googled around to look for evidence as to why they might do this but came back empty handed.
This seems so backwards to me. Why would they recommend adding them as a subdomain rather than the top level?
(I want them to use the From: xxx@topleveldomain.com, not From: xxx@sub.topleveldomain.com)