Hi there,
You can use Outlook's built-in Advanced Find feature:
1. Select the mail folder, then click on the Search input box, and select
Search Tools >
Advanced Find.
2. In the pop-up dialog, go to the
Query Builder tab, then click
Field >
User-defined fields in folder, and select the filed your custom field.
3. At last, set Condition and Value, and then Add to List, and click Find Now.
Amanda