Simon:

Is it possible to set the maximum recipients for an e-mail ? If so, any pointers please... Just I've got users sending a half meg file to 200 or so mail addresses!!

Possible yes, but easy? No, not when I last tried. If your lucky, someone will have a quick HOWTO on that one, but I'm not sure that many people still use sendmail if they have a choice. Maybe I'm wrong.

Limiting mailbox size.. I believe it's done using quotas? Can anyone point me to a good resource to figure this one out.... Is there any other way other than with quotas.....

If there is some support for it in the MTA, then you could do it that way, but that would probably be less certain than using quotas. To get started with quotas, try http://webdocs.essex.ac.uk/LDP/HOWTO/mini/Quota.html

I've been told that qmail does all of the above without any problems.. Any views on this ?

Sounds like the normal rubbish qmail apologists say without thinking. Any MTA will do quotas and you could force a limited number of recipients with any MTA. It just depends how many hoops one has to jump through. If you do use qmail, remember that it is restricted (non-free for Debian) and realise what you're getting into.