When DDEI receive emails containing header addresses without "@", it appends its hostname to the original email addresses.
Original mail header:
Therefore, the recipient will see such strange email addresses when reading the mail in the mail client software.
The original mail header is violating RFC because an email address must contain "@" and domain name. When such mails pass through DDEI, DDEI will make correction on these headers by appending its hostname.
To resolve the issue, ask the original mail senders to properly configure their mail client software, or configure the devices who send out the problematic mails, so that these mail client software or device can send emails with correct email address in headers.