It is normal to receive the NO-DOMAIN error since the original sender will be the one to be seen by TMEMS that sent the email, and since that sender is not registered to TMEMS then it will be rejected as TMEMS Relays are not open relays. For security purposes, it will only accept emails registered to it.

As an example, ExternalSenderA.com sends an email to RegisteredTMEMS.com and will auto-forward emails to ExternalRecipientB.com. In this case, TMEMS will see ExternalSenderA.com as the one sending the email to ExternalRecipientB.com.. Since it is not registered to the platform, it will be rejected.