I also found some ideas/documentation (which I have not read yet) at:
http://llug.sep.bnl.gov/doc/mrtg/examples/mrtg-mail/
http://www.jsw4.net/resources/mrtg-stuff/examples.html
http://spark.student.utwente.nl/doc/mrtg/examples/mrtg-mail/ (This is included with Debian under /doc/mrtg/examples/mrtg-mail/)

(I would also like to be able to do this with Exim and Postfix. If you have ideas, please share them.)

I am using mrtg-mailstats (available from http://mail.mandli.com/~jrowan/mrtg-mailstats.html), MRTG version 2.8.12 and mailstats (which comes with Sendmail).

On the system running MRTG, I have the following cron jobs:

*/5 * * * * root /usr/local/mrtg/mrtg /usr/local/mrtg/mail-mrtg.cfg 2>&1 > /dev/null
*/5 * * * * root /usr/local/mrtg/mrtg /usr/local/mrtg/mail-mrtg-msgs.cfg 2>&1 > /dev/null

WorkDir: /usr/local/mrtg/docs

# gather statistics from mail server with the mailerstats
# program running on port 2525
# count bytes instead of messages

Target[mail.remote]: `/usr/local/mrtg/mrtg-mailstats -b -s mail.server.here -p 2525`
MaxBytes[mail.remote]: 1250000
Title[mail.remote]: mail.server.here Mail Traffic (bytes)
PageTop[mail.remote]: <font size="+1"><b>mail.server.here Mail Traffic</b><br>\n
 Bytes per second</font><br>\n
 <font size=2>Maintainer: <a href="mailto:postmaster@mail.server.here">postmaster@mail.server.here</a></font><br>\n

YLegend[mail.remote]: Bytes per second
WithPeak[mail.remote]: ymw
ShortLegend[mail.remote]: b/sec

mail-mrtg-msgs.cfg

WorkDir: /usr/local/mrtg/docs

# gather statistics from mail.server.here with the mailerstats
# program running on port 2525
# counting messages instead of bytes

Target[mail.messages]: `/usr/local/mrtg/mrtg-mailstats -f 60 -s mail.server.here -p 2525`
MaxBytes[mail.messages]: 999999999
Title[mail.messages]: mail.server.here Mail Traffic (Messages per minute)
PageTop[mail.messages]: <font size="+1"><b>mail.server.here Mail Traffic</b><br>\n
 Messages per minute</font><br>\n
 <font size=2>Maintainer: <a href="mailto:postmaster@mail.server.here">postmaster@mail.server.here</a></font><br>\n

YLegend[mail.messages]: Messages/minute
WithPeak[mail.messages]: ymw
ShortLegend[mail.messages]: Messages/min

On the mail server, I use inetd to run mailstats. My /etc/inetd.conf file contains:

#run mailstats on port 2525
smtp-stats      stream  tcp     nowait  nobody  /usr/libexec/tcpd       /usr/sbin/mailstats

smtp-stats      2525/tcp        snmp-stats