I am not sure of anything after spending a week trying to figure this out. Here is what I read:Are you sure that mx00.ionos.com can be the correct MX entry? Should that not point to your server instead?
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Name servers are the servers that assign a specific IP address to each specific domain name that has set those name servers in its DNS settings.
They are responsible for your domain being found under a fixed IP address when the visitor types in the domain name itself, making it easier to find websites by name rather than by a set of numbers. IONOS uses its own name servers to automatically assign an IP address for a domain during the registration process at IONOS.
By default, all name server settings are pre-configured for your IONOS domain. You can configure a website, blog, or online store, and set up the appropriate email mailboxes. You don't have to worry about a thing!
Typical name server entries are:
ns1ZZZ.ui-dns.de
ns1ZZZ.ui-dns.biz
ns1ZZZ.ui-dns.org
ns1ZZZ.ui-dns.com
ZZZ represents a number between 016 and 126.
And this link:
Adjust MX Records for Receiving Email via IONOS Mail servers - IONOS Help
says:
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Note:
The DNS settings described here apply to IONOS.com customers. Different settings apply to customers with IONOS contracts in other countries.
Please note that as of 14 September 2020, receiving emails will only be possible using the correct MX record entries mx00.ionos.com / mx01.ionos.com (alternatively mx00.1and1.com / mx01.1and1.com).