Spam takes on many forms and one of the most annoying manifestations is when a friend or co-worker constantly forwards messages, many of which are hoaxes. In one case I can think of, the sender didn't know how to use the Bcc field and put everyone's address in the To or Cc field. On that fun afternoon, one or more of the recipients didn't want to get junk mail and without thinking, hit Reply to All and asked to be removed from the list. Yep, it makes for a fun day for most network engineers.

When a message like this is sent to just a few people, the mail load from hitting Reply to All won't overload most mail servers. However, when the distribution list gets up in the hundreds or thousands and even just one percent use Reply to All, it results in a flood of messages hitting the mail server.

Fortunately, most mail servers, including Exchange Server and Windows 2000 Server's SMTP service, allow the administrator to limit how many recipients can be included in any one message.

The default for Exchange 2000 is 5,000 email addresses. This setting is found in the System Manager, under Global Settings | Message Delivery, the Default tab of the property sheet. The number is total for the message, not per each To, Cc, or Bcc field. This Exchange setting applies to all addresses on the message, both internal to the Exchange server and those delivered over the Internet, as well as those contained in distribution lists.

For most organizations, 5,000 is way too high for the average user. Some departments or people might have a legitimate reason for sending larger distribution lists. You or your administrator can set limits on each user account in the Active Directory. This way, most users can have a low but reasonable limit, possibly limited to the number of users within their department or workgroup and others, such as the HR department might have a much higher limit.

The default for Exchange 5.5 is also 5,000 recipients per message. It's not quite as easy to change the limit with Exchange 5.5, since you need to add a registry setting on the server. A Microsoft Knowledge Base article, Q126497, describes how to limit the number of recipients per message, for Exchange server versions from 4.0 SP1 to 5.5. It's at:

http://support.microsoft.com/default.aspx?scid=KB;EN-US;q126497

The SMTP server in Windows 2000 Server has a default limit of 100, but it only applies to external mail. The limit for the SMTP server applies to all users and can't be adjusted per user.

There is no way to set a limit within Outlook and I'm not aware of any add-ins that allow the user to control it, although if you do happen to know of one, don't hesitate to send me an email about it. That said, might be a bad idea to let your users control the number of recipients from the client side -- using a server-side method is much safer.