We send a testing email with photo
Domain A (Exchange Online) -> eFa -> Exchange On-prem -> email become plain text
Domain A (Exchange Online) -> Exchange On-prem -> email is HTML
Other Domain (Exchange Online) -> eFa -> Exchange On-prem -> email is HTML
It is only happen when a sender domain send email to us, all email become plain text. But after remove eFa it become normal.
Other sender domains has no issue with eFa.
I have no idea how to troubleshoot the issue. I have checked that email converted to plain text in MailScanner.conf option all set to NO