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Issues with Spotlight search of Mac can also cause problems with Mail's search also. We can re-index Spotlight and see if it works or not. Close Mail. Go to System Preferences and select Spotlight. Click on Privacy tab. Under Privacy tab, click the + button and this will bring Finder window. You will see your hard disk Macintosh HD in left column under Devices section. Click it and select Choose (or select Macinotsh HD at the top drop down menu at the top middle of that window). Click Choose button.
Click OK. Now just remove it again by clicking the - button after selecting Macintosh HD. This will re-index Spotlight. Click on the Spotlight button at the top right (search button) and this will show reindexing. Let it complete.
Once it is completed, open Mail. Click on Mailbox and select Rebuild Mailbox. Try a search.
Please let me know if that didn't work, so we can continue troubleshooting.
We can rebuild the mail envelope index. Close Mail. In Finder, hold down Option key then click on Go menu and select Library. Open Mail folder, open V2 folder, open MailData folder and trash/delete every file that begins with “Envelope Index”
If you don't see these files in V2 folder, then open the V3 folder in Mail folder, open MailData folder and trash/delete every file that begins with “Envelope Index”
Open Mail. Please let me know if that didn't work