Neal,

I've submitted this as a feature request for you.

I don't think this is a "feature request" this is a bug. As I've been setting up my Exchange Server I believe System Admin messages are supposed to be sent from "postmaster@domain" so it appears SM is using a null sender when it should be a postmaster address for these messages.

Yep yep yep. I had similar issue. Client had mailbox set forwarding to another email address in the same domain where forwarding address didn't existed anymore and also had enabled autoresponder set to on. This caused some crazy loop which causes to deliver A LOT of System Administrator messages. Now when forwarding has been disabled and no System Administrator messages are being delivered I wanted to clean out client account and I couldn't do that and I can't mark messages using some global marking like mark from first occurence to last because client has some valid messages between administrator messages.

I've tried to find "System Administrator" using search function with specific conditions and this messages just can't be found. Reindexing also is not fixing this problem.

So yes. This is a bug and not a feature request.

EDIT: It looks like my issue was more related to this topic:

https://portal.smartertools.com/community/a92240/problem-with-spam-mail-that-i-cant-block.aspx 

Yes, since latest updates, forwards to either full or unexisting mailboxes create loops that fills mailboxes with ten of thousands of System Administrator messages, that aren't searchable using these keywords.

What I do to work around the search problem is find them using a keyword that would only exists in the body of these messages, like for example the delivery error reason.

I was able to fix this issue with our indexing code, it will be in our next minor. The gotcha with this fix is that it won't retroactively apply to emails that were already indexed. Reindexing a mailbox should fix it, otherwise only NEW emails will be found when searching for a sender/email address with only a display name.