eM Client 8 can use server-side search for all Exchange accounts and also IMAP accounts whose servers support this feature. This way you can get more complex results quicker without having to download your messages for offline use.

For IMAP accounts your mail server needs to support these two extensions:

• RFC 7377: IMAP4 Multimailbox SEARCH Extension (https://tools.ietf.org/html/rfc7377)
• RFC 6203: IMAP4 Extension for Fuzzy Search (https://tools.ietf.org/html/rfc6203)

You do not need to enable any setting for server-side search to work, so if it does not seem to behave as expected, check with your server provider or your admin if these extensions are available for your account.