how to renew a security certificate?

["Robert P. J. Day" <rpjday@crashcourse.ca>]

  i'm hoping this is an easy one, even though i'm going thru the docs
as we speak.  on a functioning debian system, for the last many weeks,
the clients who have fired up their thunderbird clients have been
told:

"mail.XXX.com is a site that uses a security certificate to encrypt
data during transmission, but its certificate expired on 7/7/2009
2:06PM"

mail is still being delivered, though, but it would be nice to make
that diagnostic go away.  i have a screen cap of the dialog box, which
makes it clear it's related to dovecot.  is there a simple recipe for
renewing that cert (something i've never had occasion to do)?  just
pointing me at the appropriate web page would be fine.  and is that
enough info to know how to solve the problem?  an expert mail admin
i'm not.

rday

p.s.  i'm assuming the certs are kept under /etc/ssl/certs.  is there
a cert examination command to display things like expiration dates,
that would let me poke at some of those .pem files and go, "aha,
that's the one with that expiration date."

--

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Robert P. J. Day                               Waterloo, Ontario, CANADA

        Linux Consulting, Training and Annoying Kernel Pedantry.

Web page:                                          http://crashcourse.ca
Twitter:                                       http://twitter.com/rpjday
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