Check whether IS_6-5_SP3_SrvPrtcl_Fix4 is applied in QA environment - which might not be there in your Dev environment. As per this fix, the smtp service checks for required input parameters and issues a proper error message if one is missing. 
Before this patch, one had to keep body field as null, to send any ‘HTML’ content as body.
If you set body field to empty instead of null, the service will run fine and send the mail as it should be.
Hope this helps.
#webMethods#Integration-Server-and-ESB#Flow-and-Java-services