Odd problem with addresses containing a dot

Hello,

I've installed tmda tarball on a redhat with qmail and vpopmail.
After some time I discovered that if an email contains a dot like

[email protected]

the tmda system is not invoked and the mail arrives in the mailbox.

I've checked the .qmail files and I can't see nothing wrong or different
from the others that work.

Has anyone ever had this issue?