1. Go to Administration > Settings > Incoming emails and put truncation separator into Truncate emails after one of these lines
2. Goto Helpdesk project > Settings > Helpdesk templates and put truncation separator into Email header
After Redmine 3.4 and higher we recommend using the following regular expression to truncate the most popular email response headers
^.*\d{4}.*\d{1,2}:\d{2}.*[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,4}.*:$
^(--|____|Best regards|Sincerely,|Regards|Kind Regards|best wishes,|Thank you and kind regards,|Best wishes,|Best,|Many Thanks,|Cheers,|Thank you,|С уважением,).*$