To fix this issue, install
Cumulative Update 11 or a later
cumulative update for Exchange Server 2013.
After you have installed the update, the default value is still 250, but you can edit the Microsoft.Exchange.Store.Worker.exe.CONFIG file to increase the limit of search results. To do that, you must add the following entries under the <appSettings> section as follows:
<appSettings>
<add key="MaxHitsForFullTextIndexSearches" value="
5000" />
</appSettings>
Note If <appSettings> doesn't exist, you will need to add it after the </runtime> tag.
The above example will increase the search items returned to 5000. The MaxHitsForFullTextIndexSearches value can be between 1 and 1000000. Care should be taken with this value as setting this too big could cause performance impact on the server.
Once the above file has been edited, restart the information store service on the server for the change to take effect.
Known issue
After you implement the resolution, unexpected behavior may happen to the results the Search-MailboxAuditLog command returns. When you run the command, the number of audit logs returned are unexpectedly capped at the MaxHitsForFullTextIndexSearches value no matter what you set for the ResultSize parameter.