Downgrade to the previous version:
yum downgrade qmail
Downgrade to a specific version:
yum downgrade qmail-1.03-18.el6.sme
If you're worried about qmail-1.03-18.el6.sme disappearing from the smeos repository, you can save a copy locally as insurance:
mkdir ~/save
cd ~/save
wget http://mirror.canada.pialasse.com/releases/9/smeos/x86_64/Packages/qmail-1.03-18.el6.sme.x86_64.rpm
With a copy of the current/old rpm in ~/save, you can downgrade using:
cd ~/save
rpm -Uvh --oldpackage qmail-1.03-18.el6.sme.x86_64.rpm
signal-event post-upgrade; signal-event reboot