How to check whether QMail is working?

This is using Redhat, with QMail working fine. Out of sudden, the qmail seems doesn't work, and even after rebooting the system. But, mail sending/receiving doesn't work. What's went wrong?
BalackAsked:
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jar3817Commented:
Look in your logs.

/var/log/maillog
/var/log/messages

those two files would be a good place to start.

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BalackAuthor Commented:
Is pointed to the correct direction...
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