When sending an email from iController to the debtor, whether through an automated action or the debtorsheet, the client is automatically linked to the email.
When the debtor replies, iController knows to which client this has to be linked, based on the line on the bottom of the mail:
"Please do not remove this line:: icontroller-d6333bca71bcff7d56f99f250ec24a58-000007093932"
When removing this line, iController will no longer know to which debtor the email belongs.