Brother MFC-420CN - Manual (Page 40)

1 - 20 INTRODUCTION
ECM (Error Correction Mode)
The Error Correction Mode (ECM) is a way for the MFC to check a
fax transmission while it is in progress. If the MFC detects errors
during fax transmission, it resends the pages of the fax that had an
error. ECM transmissions are only possible between fax machines
that both have this feature. If they do, you may send and receive fax
messages that are continuously being checked by the MFC.
Your MFC must have enough memory for this feature to work.