Message contains invalid header

[Marten Lehmann]

Hello,

from time to time we are getting this message in our exim logs:

LMTP error after end of data: 554 5.6.0 Message contains invalid header

I have also experienced this error while I'm syncing emails from an old 
server to our new cyrus mailserver. I have

munge8bit: false

in my imapd.conf and reject8bit is set to "false" by default. Which 
headers is cyrus complaining about? I cannot change this world where 
clients like Notus Notes are sending invalid emails, but simply refusing 
these mails is a bad choice. And we also have a lot of emails we 
received in the old setup and I wouldn't know how to explain to our 
users, that they cannot access these messages any more.

How can I make cyrus less strict?

Regards
Marten