Hi Murugan,
Could you please confirm whether your query is why automatic TO is not generated in spite of customization settings are in place? or do you want to know what customization settings has to be maintained to generate automatic TO?
If your query is why automatic TO is not getting created, then I would request you to maintain mail control customization which would be helpful to find out why exactly TO is generated. Please refer this KBA 1572908 which would be helpful to maintain it.
Thanks,
Balaji