So, looking at your example, it appears you're doing a Message Archive Search for a message from synology@infinity.com.my to your alias: itsupport@infiginty.com.my. Is that the case?
Let me share the 2 result of the share. 1 is to itsupport@, 1 is to tommytan@
I search 1 by the subject and from synology@... only. It also show up 4 only so i think your logic make sense.
So I do another seach from synology@... to tommytan@...., it show 0 result.
I do from synology@..., it show 27 result.
So the previous version did not archive correctly.
I change the version of my smartermail in the month of September but unfortunately I do not remember which version I come from. My 1 months archive for Aug is 126GB, For the month of Sept is 1.14TB.