For any error start with Basic Troubleshooting to find out the error details.
In this case I think I can guess the cause of the error. If you look in the Web.config fille in the root folder of the site you will probably find this:
<!-- this isn't used anymore for anything the only smtp settings used are in the <appSettings section so you can move them to user.config -->
<network host="localhost" port="25" password="" userName="email@example.com"/>
it is true as the comment says that the settings from this section are not used, but we have found that the section still must be there or it causes an error, so the problem is that this section is commented out and it needs to be changed by removing the bold parts <!-- and --> from around the section.
Again the settings there don't have to be correct but the section must not be commented out.
Hope that helps,