(Using Kontact Version 4.14.10, but the issue is not new to that version)

If I go to "Find Messages" and specify a search criteria of "Subject" "does not contain" "some string", my result set contains *only* results that contain "some string"

Let's say I have a message with a subject of "[Backups 0017611]: somehost - Backup - 43251 - OK"

If I search for "Subject" "does not contain" "Backups", that message will appear in my result set. Likewise, if I search for "Subject" "equals" "Backups", that message will appear in my result set. 

Reproducible: Always

Steps to Reproduce:
1. have a message with a subject of "[Backups 0017611]: somehost - Backup - 43251 - OK"
2. search for "Subject" "does not contain" "Backups", that message will appear in my result set. 
3. search for "Subject" "equals"Backups", that message will appear in my result set. 

Actual Results:  
"[Backups 0017611]: somehost - Backup - 43251 - OK" appears in result set

Expected Results:  
"[Backups 0017611]: somehost - Backup - 43251 - OK" should not appear in result set.

The lack of these search mechanisms makes searching my mail very difficult...