The Clam AV developers have announced that "ClamAV installations older than 0.95 will be disabled" on 15th April (
http://www.clamav.net).
This will have an impact on SME server installations only which are NOT updated and email deliveries will stop working on those! There are a few forum messages and bug reports indicating this issue and for all of these it appear that these SME server installations were NOT up to date.
If there is any doubt about your email server working properly, update your SME server installation first before reporting any issue.