SME Server uses ClamAV to do virus scanning, the current actual version for SME Server 7.x is 0.96.
Servers that are running SME Server 7.x and do still run a version of ClamAV which is 0.94 or lower, or servers that are running releases of SME Server previous to SME Server 7.x might experience problems with mail delivery when virus scanning is enabled.
This functionality change is outside the scope of SME Server development team and the decision to do so is the sole responsibility of ClamAV, hence we can not active older releases of ClamAV.
The solution is to update your server to the latest release of SME Server 7, which includes ClamAV version 0.96. If you do not want or cannot upgrade email delivery might fail until you disable virus scanning for email delivery.
More details on the what and why of disabling ClamAV releases previous to 0.94 can be found on the ClamAV website:
http://www.clamav.net/lang/en/2009/10/05/eol-clamav-094/