Hi Marcel
As for the issue at hand: the DNS servers of your customer are (were) not properly reachable from our IP ranges or more specifically we are (were) unable to get the IPs for ns1.aerni.com or ns2.aerni.com. During our investigation this resolution has mostly recovered and we are able to resolve the domain properly, so this issue should now be resolved. This makes it harder though to pinpoint the exact cause here, to us it currently seems though that NS records have been changed and the old DNS servers been turned off before the TTLs of the old NS records had expired, so some intermediate DNS/CNS servers still had cached info that was now no longer working and this made the domain unresolvable to our servers.
As a side note: both NS entries now share a single IP and that is even the same IP as the mail server, this is definitely not an optimal setup, an offsite secondary DNS server that didn't need changing might have prevented this from the start.
Ok, thank you, this clarifies and explains the issue. Yes, the customer has troubles with his main internet link at the moment and uses this ip address as his backup link.
I know he made mail.aerni.com pointing to the backup IP to be able to receive emails over the backup link.
I guess he also changed the NS entries at his registrar, so his DNS Servers can be reachable at this IP for the time of the outage. I'll suggest to him that he should have a secondary outside of his network so DNS is not dead if his link dies.
Mit freundlichen Grüssen
-Benoît Panizzon-