The code is doing what its supposed to do and trying to send the message, if it is failing it is failing for reasons beyond the control of mojoPortal.
"The server response was: SMTP authentication is required."
Sounds like something with your smtp server configuration but I'm not sure exactly what. The error sounds like possibly something with the from address needing to authenticate in order to send mail through your smtp server, but if its only happening with some messages I have no idea why unless different from addresses are used for some of them.
Best,
Joe