The GMX error 550 invalid DNS MX or A/AAAA resource record can indicate a DNS problem with the sending domain.

In one documented case, the MX record pointed to a hostname that was itself a CNAME alias for mail.edis.at.

The fix in that case: Point the MX record directly to mail.edis.at. Use this destination only if EDIS actually handles incoming email for your domain.

An MX target must not be a CNAME alias and must have appropriate address records. See RFC 2181, section 10.3.

For other configurations, ask EDIS support to review the complete error message and your DNS records.