[Mailman-Users] sudden problem with mailman sending out traffic

[Bruce Harrison]

Mailman has been running well for months.  All of a sudden no traffic is not going out.  /var/log/maillog shows no errors.
Mailman's smtp-failure shows:

Sep 19 13:31:32 2011 (1215) delivery to wilbcros at ut.utm.edu failed with code -1: (101, 'Network is unreachable')
Sep 19 13:31:32 2011 (1215) delivery to briabrew at ut.utm.edu failed with code -1: (101, 'Network is unreachable')
Sep 19 13:31:35 2011 (1215) Low level smtp error: (101, 'Network is unreachable'), msgid: <mailman.6.1316457053.1213.junk-l at mailman.utm.edu>
Sep 19 13:31:35 2011 (1215) delivery to harrison at utm.edu failed with code -1: (101, 'Network is unreachable')
Sep 19 13:31:38 2011 (1215) Low level smtp error: (101, 'Network is unreachable'), msgid: <mailman.7.1316457053.1213.junk-l at mailman.utm.edu>
Sep 19 13:31:38 2011 (1215) delivery to junk-l-owner at mailman.utm.edu failed with code -1: (101, 'Network is unreachable')
Sep 19 13:31:47 2011 (1215) Low level smtp error: (101, 'Network is unreachable'), msgid: <E6C16961ECB6B044A889CDAC6BA7B4B23CE035E9 at EXCH2010MBOX1.utm.edu>
Sep 19 13:31:47 2011 (1215) delivery to harrison at utm.edu failed with code -1: (101, 'Network is unreachable')
Sep 19 13:31:47 2011 (1215) delivery to afortner at utm.edu failed with code -1: (101, 'Network is unreachable')
Sep 19 13:31:47 2011 (1215) delivery to tlewis at utm.edu failed with code -1: (101, 'Network is unreachable')

I can ping and telnet to the upstream mail agent.
This seems to have just started happening this morning.  Upstream mail agent is an exchange system front end.  Exchange is running fine, etc.
Exchange guy is looking at logs...

Any ideas anyone ?