About a week ago we suddenly started getting this error: Registration at CallCentric has failed. Destination (sip:firstname.lastname@example.org:5060) is not reachable, DNS error resolving FQDN, or service is not available. This eMail was send from an 3CX SMTP Relay Service. If you suspect the service has been used unlawfully and been used outside the scop of delivering 3CX Phone System related e-mails file an abuse report here: https://www.3cx.com/smtpabuse We are hosting the PBX on Google Cloud, and had not experienced this before. I am not sure where to even start troubleshooting. When this happens, we cannot get calls, but resolves by itself after a few minutes. Ideas? Thanks in advance.