Just to clarify, you sent an email from (example @gmail.com) to invalid.user@yourcompany.com. In that case, the NDR will NOT be generated by Xeams but @gmail.com. NDRs are generated by the SMTP server that accepts the message. Consider the following example:

                    1                    2
User ------> User's SMTP Server -----> Xeams
       ✔                         ✖       

Two SMTP servers are involved: 1 and 2 (Xeams). In this case, the user's SMTP server accepts the message but not Xeams. Therefore, generating an NDR is the responsibility of server 1, not 2.

Check SMTPConversation.log to confirm an incoming email was rejected by Xeams. Search for the sender's email address to locate the log entries.