Beta 8 server-side search

How do we enable or use server-side IMAP searches?

For Beta 8 queries / problems send an email to testing@emclient.com

If it is any help, 8.0.1256 allows enabling/disabling server search.

No - it doesn’t. eM Client has not implemented standard IMAP sever-side search. They decided to implement something Exchange-specific - so unfortunately it is not compatible with a properly configured Dovecot server with FTS search.