I found this quite by accident. I have two licensed versions on eM Client running on two computers, and this worked to kill the app instantly on both.
dia:beacon is the name of a museum (that’s how they write it) in Beacon, New York. I wrote “dia:beacon” in my search box. I set the search to “current folder” and that folder was “sent”, I tell search to look for this string in “body”. Then I click “advanced search” and eM Client drops dead.
I thought you’d like to know. It comes back eventually after checking the databases.
DGI
Hello David,
We’re very sorry for the inconvenience. We are however aware of this bug and it will be fixed in eM Client 7.1.
Thank you for your patience,
Maurice
@Maurice Hester,
When is 7.1 slated to come out? Is there an announcement somewhere?
Thanks for letting us know! 
Hello,
We’re currently in the alpha testing stage of 7.1, doing our best to ensure the product’s quality. The release will definitely be announced on our blog (http://www.emclient.com/blog) / facebook (http://www.facebook.com/eMClient) / twitter (http://twitter.com/emclient).
Regards,
Maurice
Thank you, Maurice. We’ll keep an eye out for further announcements on 7.1.
Thank you, Maurice. This (for me at least) is more an interesting/odd discovery than a burden. I appreciate your attention to it!