Hello Senthil,
Try debugging by the webpage. This error normally pops up when the mail API is not receiving valid email address of the recipients like "". Try if the mail address are correct by putting an alert just before triggering the transaction.
Also, you could if correct inputs are getting assigned to correct transaction input parameters. Make sure there is no case sensitive issues.
Incase, you still have the error, you could post a snippet of script.
Regards
Tufale Ashai