SME unaltered is not an open relay. One of the tests shows that it might be with SME5.5, though it is a false positive. SME5.6 removes the false positive.
Mitel's servicelink has an antivirus built in along with other features, you will need to contact them for more info.
Another option is to purchase RAV through Darrell May at myezserver.com
There also may be free AV apps that you can fiddle with, a search of the forums will give you lots of information.