Hi,
Sorry for the delay in responding. Had some email configuration issues myself on this site and did not get notification for this post and then was out of town for the holidays.
Its posible that you accidently removed a clsoing tag or somethign while editing the Web.config. I would back it up and then restore the original from the download and then make sure it works before editing it again.
Another thing that can cause this type of error is if you have mojoportal installed in a sub folder and there is also a Web.confg file in the folder above it. For example if you have it running under localhost/mojoportal and there is another web.config file in the folder corresponding to localhost it can cause conflicts.
The orignal error may have been caused by the sender/return address of the message not being alloowed by your smtp server. So you need to make sure the default from address is from the correct domain.
Hope it helps,
Joe