How to properly empty log files

[T o n g <mlist4suntong@yahoo.com>]

Hi,

My /var/log/mail.err is very big due to a misconfiguration error. 
However, I can't simply remove it because it is "owned" by rsyslogd:

$ lsof | grep mail.err  
rsyslogd   4387       root   10w      REG        8,9  17605899     
247416 /var/log/mail.err

So, how to properly empty log files?

thanks

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