[CentOS] Why are dots in username & groupname no longer allowed?

For some strange reason I am no longer able to create groupnames or 
usernames (with useradd and groupadd) that contain a dot.

Uptill now it was possible to create user&groupnames like c.smith but now 
I get these errors:

[root at server etc]# useradd c.smith
useradd: invalid user name 'c.smith'

[root at server1 etc]# groupadd c.smith
groupadd: c.smith is a not a valid group name


Anything without a dot works without problems.

The box is a (fully updated) CentOS 3.6 x86_64 box with Dual Opteron cpu.

Thanks for any hints!