hi Charith
If you are able to trigger the email correctly to the vendor then there is no problem in your config. You are using expiditer. For this the standard output type would be MAHN
Please check in your PO messages what is te output type trigger, is it ZNEU (unlikely) or MAHN or some other Z. If it is something other than ZNEU, please ask your abaper to check the abap forms for that output. It is likely that there is no message defined in the form
Also, if these dont solve your issue, parallely you can paste screens of your message history of PO and what is the problem you see in the email
Regards
Vivek Singh