The problem is qutite simple to understand - have you ever sent an email CC'd to many recipients, but one of the recipient's email no longer exists, and the email delivery stopped in the middle of a long CC list?

That's my problem... I have some 400 email addresses but there are a couple of them (hard to find manually) which no longer exist. When i include these 400 recipients in the CC or BCC list to send the email to all of them, if the non existant recipient happens to be the 4 th one in the CC list, only the first three recipeints receive the email, and then the delivery stops...

How can i deal with this problem? Is it client (eg, Outlook express a la windose) or server oriented?

What client are you using to send your mail? On most UNIX systems, it's the Mail Transfer Agent (eg. sendmail) that deals with this, and sendmail does the right thing, so I suspect it's in your client. Some clients might send the message multiple times to the MTA, and stop on the first failure. I need to know more about your setup to help, such as your mail client, OS version, distro, if you're using sendmail or not, and so on.

This was really a general question, not necessarily on Linux systems. For example, Microsoft's Outlook Express. I haven't tried it with sendmail, but i'm sure it will give no problems, but I haven't encountered this situation before (I was simply asked by my friend who uses windose)

Oh. Well, it's all up to the client as to how it sends a message out. Some clients (like MSOE) are also Mail Transfer Agents, so they do the actual sending of the mail. MSOE probably does stop on the first error. That's the kind of crappy error handling we've come to expect from Microsoft. There's not much you can do in that case, except to remove the addresses up to and including the failed one and try again.

The real answer is to get a mail client that gives you some control over this, or at least makes more of an attempt to deliver your mail and handle delivery errors better.