From the draft 1.8b release notes:
 
- (non-VM) New SMTP_RESET_EVERY configuration option: this option directs
  LISTSERV to  drop its outbound  SMTP connections at  regular intervals.
  The value of  the keyword is the number of  minutes between each forced
  timeout. This option may be useful if  you are using sendmail as a mail
  delivery agent.  On an active  system, there  may be enough  traffic to
  keep the  connections active 24 hours  a day. The size  of the sendmail
  process driving the connection appears to grow with time, with observed
  values of a dozen megabytes after a week of activity, presumably due to
  storage fragmentation.  Since this  is the process  from which  all the
  delivery  subprocesses are  forked, the  sendmail processes  get bigger
  with time, and  some customers have reported increased swap  rates as a
  result. Resetting the connection regularly (say every 8h) prevents this
  phenomenon.
 
I will update this paragraph with your figures :-)
 
  Eric