On Thursday, October 9 at approximately 19:55 CEST (17:55 UTC, 13:55 EDT, 10:55 PDT) a configuration change was implemented on the incoming email servers to verify the syntax of headers in incoming email and reject messages with invalid headers.
However, a part of the configuration was incorrect which resulted in email with valid syntax being rejected back to their sender.
The error was discovered and reverted at approximately 22:05 CEST (20:05 UTC, 16:05 EDT, 13:05 PDT), after which incoming email started being delivered correctly.
We apologize for the incident and are in the process of contacting the customers affected by this issue with further details.