Add your records at IONOS

When IONOS runs your domain's DNS.

Add the records

Go to Domains & SSL, select the gear icon under Actions next to the domain, then DNS.
If the MX records belong to an IONOS mail service, disable it first: in the Service column, select the gear icon, then Disable service.
Select Add record, choose the type, and enter the host name and value from your domain's page. Use @ for the domain itself. The MX record's priority has its own field. Save each record.

IONOS gives every domain an autodiscover record of its own. Edit it to point to autodiscover.postpike.net rather than adding a second one. Leave the _domainconnect record alone.

From IONOS: using another provider's mail servers, IONOS: managing TXT records. Menus change: if something here doesn't match what you see, tell us.