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we are encountering issues with some users getting error notifications after replying to an email from an external partner:

554 5.6.0. Bounce, id=01761-13 - bad header
cause: "mime version" occurs 3 times

if we check the mail header it does contain "mime version" 3 times

Lotus Domino 9.0.1. FP5
Traveller 9.0.1.8

I always assumed this information cam from the mail server and is not altered by the mail client. But can anybody provide me with an overview of this protocol?