The error message from Gmail was misleading. They refused the message because of the combination of wrong SPF entry and missing Message-ID. After setting the correct SPF entry, the missing Message ID is obsolete, and the message arrives in Gmail. Can you give me an example how the filter script regarding the Message-ID should look like?
Mails to Gmail - message-id header is missing
Suppress progress logging
New Windows version
option to disable progress of transfer
Save date to log file