Virtualmin Theme version: 6.7
Virtualmin version: 3.66
Operating system: Ubuntu Linux 8.04.2
After a clean install using the instal script, I found that users couldn't send mail using SMTP since authentication failed. As it turned out, the user postfix was ran under (postfix) was not member of the group sasl.
adduser postfix sasl
That solved it, but I think the virtualmin config check should include that, too.