Recover mail lost due to distribution list rejecting mail from non-authenticated senders

Hi,

I have a bit of a situation, hoping to reach out as google isn't getting me anywhere on this topic.

We recently gave out a smtp address tied to a distribution group which was accidentally configured to only allow messages if senders were authenticated. A few dozen external customers sent emails which were processed by our email gateway, andsuccessfully delivered to Exchange which was a surprise to me.

Once in the Exchange environment, I can see tracking logs showing its path as below:

RunspaceId              : d6fee167-5fa3-4d58-9354-b10402c9dacf
Timestamp               : 4/29/2016 9:53:34 AM
ClientIp                : 10.10.X.X
ClientHostname          : GATEWAY1
ServerIp                : 10.11.X.X
ServerHostname          : EXCHANGE1
SourceContext           : 08D36FD672846E86;2016-04-29T13:53:34.862Z;0
ConnectorId             : EXCHANGE1\Exchange Connector
Source                  : SMTP
EventId                 : RECEIVE
InternalMessageId       : 13683389
MessageId               : <bb3a65e077043968b99b37d681fc2e48@mail.hotmail.com>
Recipients              : {distribution_list@internal.local}
RecipientStatus         : {}
TotalBytes              : 41037
RecipientCount          : 1
RelatedRecipientAddress :
Reference               :
MessageSubject          : Yadda yadda
Sender                  : externaluser@hotmail.com
ReturnPath              : externaluser@hotmail.com
MessageInfo             : 00A: NTS:
MessageLatency          :
MessageLatencyType      : None
EventData               : {[FirstForestHop, EXCHANGE1.internal.local]}
RunspaceId              : d6fee167-5fa3-4d58-9354-b10402c9dacf


RunspaceId              : d6fee167-5fa3-4d58-9354-b10402c9dacf
Timestamp               : 4/29/2016 9:53:35 AM
ClientIp                :
ClientHostname          : EXCHANGE1
ServerIp                :
ServerHostname          :
SourceContext           :
ConnectorId             :
Source                  : ROUTING
EventId                 : FAIL
InternalMessageId       : 13683389
MessageId               : <bb3a65e077043968b99b37d681fc2e48@mail.hotmail.com>
Recipients              : {distribution_list@internal.local}
RecipientStatus         : {550 5.7.1 RESOLVER.RST.AuthRequired; authentication required}
TotalBytes              : 41037
RecipientCount          : 1
RelatedRecipientAddress :
Reference               : {<34af8566-3405-4204-95d1-b0f0703e8280@internal.local>}
MessageSubject          : Yadda yadda
Sender                  : externaluser@hotmail.com
ReturnPath              : externaluser@hotmail.com
MessageInfo             :
MessageLatency          :
MessageLatencyType      : None
EventData               :

Message tracking doesn't show these items as existing anywhere. I haven't done a discovery search yet, planning on it but not expecting a result. Our gateway doesn't retain mail if it succesfully hands it off to Exchange, which it did.

Have these mails disappeared into thin air? No bounceback went out. No notification to the destination user or receiving user.

If anyone has dealt with anything like this before, please let me know if you found a way to recover these messages.

Thank you,