First, you are using avast! Server Edition, right? (NOT the Professional Edition).
Second, have you completed the Server Deployment Wizard that poped up after you rebooted after installation? If so, you were asked whether this is a Terminal Server or not. I guess you selected 'not a terminal server', and clicked Next, and you were given the chance to enable the desktop providers. It seems to be that you have enabled the Internet Mail provider although you're running on SBS.
This is unnecessary. The Internet Mail provider is meant for desktops only. To protect Exchange, you would use the server plugins (the Exchange 2000 and SMTP 2000 providers).
So to make long story short, you can rerun the the Server Deployment Wiz (it's in the avast group of Start menu), select Normal server, and not select the Internet Mail provider; from the same page you may also start the 'Mail Protection Wizard' and choose Remove protection from all mail accounts.
This should completely get you rid of the Internet Mail provider that is displaying this warning. It's no really wonder that it is displaying it in your case because it works as a local SMTP/POP/IMAP proxy and you already have Exchange installed on that machine so the ports are already occupied.
Hope this helps,
Vlk