The above is pretty harmless. Some visitors to your webmail URL have the google toolbar installed. The toolbar is apparently confused and trying to load the favicon for the X3 webmail. As you noticed, there are two different versions of the toolbar doing this. That likely means there are two different comptuers, behind a firewall or proxy, that are accessing webmail.
Based upon the log snippets you provided, this is a likely scenario ( using example.com in place of your website):
1. Person1 accesses example.com/webmail
2. Because /webmail is valid, a 200 status is logged in the domain log for example.com
3. The google toolbar gets confused by something in the server response ( some conjecture here as hard data is not available )
4. The google toolbar attempts to fetch the favicon for webmail, using an invalid URI. This results in the 404 entry in your domain log
5. Person2 accesses example.com/webmail
6. Steps 2 - 4 are repeated for Person2, who also has the google tool bar
It looks like both requests are originating on AT&T's network from Milwaukee, Wisconsin ( i.e. milwwi.sbcglobal.net ).
One thing to keep in mind is any domain on a cPanel server will serve up the webmail interface, regardless of whether the email account is associated with the domain or not.