New error when emailing invoices

Suddenly getting this error when we try to email a sales invoice.

“GSSAPI operation failed with error - Unspecified GSS failure. Minor code may provide more information (Cannot find KDC for realm “redacted.com.au”)”

I haven’t been able to find any information on this error. Could anyone shed some light on it?
Cheers

I’m going to mark this as a bug for now because it could have something to do with transition to .NET Core recently.