[Mailman-Users] no email sent - need help

[weweje network]

Hello, list.
I'm kind of new to server admin, but not despaired ;)
I'm using mailman on CENTOS+PLESK
Everything is perfect except no mails are sent.
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[root at xxx mailman]# /etc/init.d/mailman restart
Shutting down mailman:                                     [  OK  ]
Starting mailman:                                          [  OK  ]
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mailman is running
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 tail -f smtp-failure

Jul 21 21:27:51 2008 (10562) delivery to ggggggggg at gmail.com failed
with code -1: (-2, 'Name or service not known')
Jul 21 21:27:51 2008 (10562) Low level smtp error: (-2, 'Name or
service not known'), msgid:
<mailman.0.1216651288.19698.list at domain.tld[edited]>
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No documentation on that type of error
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Can someone suggest a path?
thanks