Based on very specially configurations emails to external recipients will be shown thousands of times in the Exchange Servers mail queue. The mail queue log files could be fill up your log file partition also. The deletion of these special email will not solve the problem, because these mails will be come back again and again.
When Microsoft Exchange Server will receive an email for an email domain that is not part of "Accepted Domains", that have to be routed to external mail servers, Exchange Server will start a DNS request for the dedicated MX Record to get the IP address and hostname of the competent email server for this domain and the email delivery.
In this case the external MX record for the domains shows the hostname "localhost" and the IP address 127.0.0.1 or none IP address.
Based on this information Microsoft Exchange Server tried to deliver the email to its own localhost and so to itself. This is of course a bug in all Microsoft Exchange Server versions! Microsoft Exchange Server is not able to detect the error when it get back an DNS request that is pointing to itself even if this email domain is not part of the local accepted domains!!
This bug is not yet fixed in Exchange 2016 CU2, therefore I have blocked emails to this dedicated external email domain whith the help of MS Exchange Server transport rules.