Fails to retrieve the source email exchange online when doing a mail migration between two tenancies

Hi All,
I'm running a email migration between two tenancies.

But i'm getting this error below for a particular user and on some particular email item
Failed to retrieve the source email. Details: Specified argument was out of the range of valid values. (Parameter 'length')

Any ideas what could be going wrong?
I'm using avepoint fly for the migration