[Dovecot] receive date wrong in outlook

hi, we migrate from exchange to dovecot. we do it with imapsync. first we try to do it with --syncinternaldates, but this fails on many messages with invalid date, so we drop this option. unfortunately after this those users who use outlook see the received date as the date we do the migration in stead of the real received date. other clients eg webmail, thunderbird etc sees the right date. so my question what can i do? it's a dovecot bug or outlook and if outlook's bug is there any workaround fro this? thanks in advance.

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