How to control mailbox's size?

Quit rude perhaps, but you could enable quota's on the partition that holds /var/spool/mail. I don't know exactly, but I think the users new mail will be dropped when the quota limits are reached.

 

Well you could apply disk quotas to the filesystem that the mail spool is on, but every document I've ever seen on sendmail says that this is a "BAD IDEA". the problem is that the sendmail+imap4 doesn't check the space free before starting to accept a message. If that user runs out of quota while the message is being received bad things happen. Also there's no mechanism for notifying the user that their mailbox is at or near quota.

The Cyrus IMAP server implements a quota system for the mailboxes at a level that's compatible with the mail system. You can find out more about it and its quota system at: http://asg.web.cmu.edu/

I don't know if there's a way to specifically limit mailbox size, but you can limit the total file space a user takes up (which would effectively limit mailbox size).  the linux quota system limits disk space by user and group and by filesystem, so if your mailboxes were on their own filesystem you could limit just the users disk space for that filesystem.  Here's the text from http://metalab.unc.edu/pub/Linux/docs/HOWTO/mini/Quota  ::

4.3.   Modify your system init script to check quota and turn
  quota on at boot


  Here's an example:

  ______________________________________________________________________
  # Check quota and then turn quota on.
  if [ -x /usr/sbin/quotacheck ]
          then
                 echo "Checking quotas. This may take some time."
                 /usr/sbin/quotacheck -avug
                 echo " Done."
          fi
           if [ -x /usr/sbin/quotaon ]
          then
                  echo "Turning on quota."
                  /usr/sbin/quotaon -avug
          fi
  ______________________________________________________________________



  The golden rule is that always turn quota on after your file systems
  in /etc/fstab have been mounted, otherwise quota will fail to work. I
  recommend turning quota on at the end of your system init script, or,
  if you like, right after the part where file systems are mounted in
  your system init script.

  4.4.  Modify /etc/fstab



  Partitions that you have not yet enabled quota normally look something
  like:

  ______________________________________________________________________
  /dev/hda1       /       ext2    defaults        1       1
  /dev/hda2       /usr    ext2    defaults        1       1
  ______________________________________________________________________



  To enable user quota support on a file system, add "usrquota" to the
  fourth field containing the word "defaults" (man fstab for details).

  ______________________________________________________________________
  /dev/hda1       /       ext2    defaults        1       1
  /dev/hda2       /usr    ext2    defaults,usrquota       1       1
  ______________________________________________________________________



  Replace "usrquota" with "grpquota", should you need group quota
  support on a file system.

  ______________________________________________________________________
  /dev/hda1       /       ext2    defaults        1       1
  /dev/hda2       /usr    ext2    defaults,grpquota       1       1
  ______________________________________________________________________



  Need both user quota and group quota support on a file system?

  ______________________________________________________________________
  /dev/hda1       /       ext2    defaults        1       1
  /dev/hda2       /usr    ext2    defaults,usrquota,grpquota       1   1
  ______________________________________________________________________



  4.5.  Create quota record "quota.user" and "quota.group"


  Both quota record files, quota.user and quota.group, should be owned
  by root, and read-write permission for root and none for anybody else.

  Login as root. Go to the root of the partition you wish to enable
  quota, then create quota.user and quota.group by doing:

  ______________________________________________________________________
  touch /partition/quota.user
  touch /partition/quota.group
  chmod 600 /partition/quota.user
  chmod 600 /partition/quota.group
  ______________________________________________________________________



  4.6.  Reboot


  Now reboot system for the changes you have made to take effect.

  Also note that subsequent partitions you wish to enable quota in the
  future only require step 4, 5, and 6.


  5.  Quota Setup on Linux - Part II: Assigning Quota for Users and
  Groups


  This operation is performed with the edquota command (man edquota for
  details).

  I would normally run quotacheck with the flags -avug to obtain the
  most updated filesystems usage prior to editing quota. This is just a
  personal habit, and not a required step however.

  5.1.  Assigning quota for a particular user


  Here's an example. I have a user with the login id bob on my system.
  The command "edquota -u bob" takes me into vi (or editor specified in
  my $EDITOR environment variable) to edit quota for user bob on each
  partition that has quota enabled:

  ______________________________________________________________________
  Quotas for user bob:
  /dev/hda2: blocks in use: 2594, limits (soft = 5000, hard = 6500)
           inodes in use: 356, limits (soft = 1000, hard = 1500)
  ______________________________________________________________________



  "blocks in use" is the total number of blocks (in kilobytes) a user
  has comsumed on a partition.

  "inodes in use" is the total number of files a user has on a
  partition.

I agree with jlevie, thats a cleaner solution. But since he said "RH6.0" i figured he wanted to stick with the "tsandard tools" Since cyrus only supports quota if you switch from unix-format to their own mbox format, which means that (for instance) Pine will not be a happy camper. But it is a better solution in the long run.