Encoding of outgoing emails of function smtp_to

Hi experts,

When I send the emails with Chinese texts with function smtp_to, some recipients can view the Chinese however some recipients view the garbled characters.

I believe this is the product issue because those recipients can receive the Chinese emails normally for others senders.

Only emails sent by the Data Services produce gargled Chinese characters.

This is not related to the complete locale.

Any method to tune the encoding for the outgoing messages?

Regards,

Gordon