Open-Xchange can make use of an IMAP servers capability for searching mails. For this, the property 'com.openexchange.imap.imapSearch' in 'imap.properties' had to be set to 'imap'. Nevertheless the server decided to perform the search on its own (in-memory), if the number of messages in the selected mail folder was below a threshold (also configurable via 'com.openexchange.mail.mailFetchLimit' in 'mail.properties').

With 7.6.0 'com.openexchange.imap.imapSearch' can be set to 'force-imap'. This forces the server to delegate the search request to the IMAP backend, regardless of the number of existing mails in the selected mail folder. The default value for this property stays 'imap'.