Hello Pranjal,
If it is user-exit then
IWO10009 - in case the mail triggers at Work Order SAVE event.
IWO10002 - in case the mail triggers at Work Order Release event
If you have verified and not found anything here, then most probably one of the methods in BAdi WORKORDER_UPDATE might have been used.
This method can be:
AT_SAVE or
AT_RELEASE
Also you should look into user-exits (Very little probability)
CNEX0026
PPCO0023
Regards
KJogeswaraRao