Outlook mail search does not work on VDAs with UPM 7.18  Native Index SearchIndexer 7.0.16299.402

Event id ID 3000 
"The Windows Search Service has been updated and not tested in Citrix"
Contact technical support team to obtain private correction to support the new Windows Search Service@

You may have to follow the steps/configure Outlook as given in this blog: 
https://blogs.technet.microsoft.com/outlooking/2014/04/28/understanding-search-scopes-in-microsoft-outlook/
 

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Problem Cause

If the shared mailbox is added via the Exchange email account’s Advanced tab (using the Open these additional mailboxes option) or via Auto-Mapping, you should be aware of the following limitation when the Outlook Search option is set to the All folders setting (you can view your Search options by clicking on the File tab, clicking Options, and then Search): all search queries are executed against all folders of the primary mailbox. This occurs because the global setting was changed to ‘All folders’. However, searching across multiple mailboxes using the ‘All folders’ scope is not supported. In this scenario, after typing the search criteria to get instant results in Cached Exchange Mode (or pressing Enter in Online Mode), the search scope displayed in the upper left corner of Outlook shows All Mail Items or All Outlook Items selected. This is true even if you initiated the search in a folder of the shared mailbox. The end result is that no items from the shared mailbox are returned. To include the shared mailbox in the scope, you need to manually reselect Current Folder. 

It is the behaviour/limitation from Outlook which restricts the search. It is explained in following Microsoft Blog: https://blogs.technet.microsoft.com/outlooking/2014/04/28/understanding-search-scopes-in-microsoft-outlook/