I was working with a customer who reported that his Outlook searches for emails from only user only went back about a month. We did some testing and found that when he searched for emails from another person, they only went back one week. In both searches, I noticed that the number of items found was 250.

250 seemed like a very round number and would have been a huge coincidence that searches for two different people would result in the same number of results found. I did some investigating and found that there is a "feature" in Outlook to limit the search results to increase the speed of searches. 

The feature that enables limiting search results is "Improve search speed by limiting the number of results shown" and is enabled by default. In Outlook 2013 and 2016, this setting limits results to 250 items. In Outlook 2007 and 2010, this setting limits results to 200 items. When this option is unchecked, it will return all items found. Also, if you click "More" at the end of the search results, this setting will be ignored as the "More" button searches on the server and will only return the limited result set.