The smtp error likely means that it failed to relay mail to one or more of the subscribers to your forums. This is most likely incirrect email addresses or mail server not available or not responsive. So its not a problem in mojoportal, its trying to relay the mail but its getting a timeout when relaying to some addresses.
The other error and variations thereof I see all the time in my own logs as well and I believe they are bots not real visitors raising those errors.
ASP.NET protects certain file extensions like .ascx and .config as these files are not meant to be requested directly by an http request. .ascx is a user control, its never requested directly, its normally contained inside a page (.aspx is a page extension it is not protected as it is expected to be requested via an http request).
Whenever a web request is made for a protected file, ASP.NET invokes the HttpForbiddenHandler which does not serve the file but raises an exception. mojoPortal is just logging this exception and this is what you see in your log.
Its possible for a user to raise this exception but it won't happen in normal use of mojoportal. You can try it your self by requesting the url yoursiteroot/Controls/StyleSheet.ascx
You will see the same errors in your log.
These bots are a big nuisance but are not causing any real harm unless you get huge volume of requests like this.