Is this a bug: user in default domain can log into same user in virtual domains

[daniel at shera.com.cn]

I made user authenticaton done against mysql via pam. The problem is the user name, say user1, and its password in default domain can be used to log into the same username account of other virtual domains by putting user1 at virtualdomain.com in client programs like outlook express. Is this a bug? If not, what is the right form to store user name of the default domain in mysql? I just put user1 because with user1 at defaultdomain.com I could not authenticate with outlook express.
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