Mailing list software

We're running a number of IETF mailing lists on our server here at
Columbia. Some of the lists have gotten a bit larger and more popular
than we anticipated; sendmail is groaning. Any suggestions for alternate
delivery software (for Solaris and/or Linux)? (Since this is somewhat
off-topic, please send me responses privately and I'll summarize if
there's interest.)
-- 
Henning Schulzrinne   http://www.cs.columbia.edu/~hgs