If you create a MIME.Message that has an array(string) for one of its headers, then you can't cast it to a string:
Unknown program: `/ (({"test1@siriushosting.com","test2@siriushosting.com"}),"\0") /usr/local/lib/pike/modules/MIME.pmod/module.pmod:1059: MIME.Message()-
__lambda_65759_3_line_1058("to")
Adam