Setting Message Size Limits (IIS 6.0)
There are two message size limit settings. The first, Limit message size to, is a preferred message limit for the virtual server. This is what the SMTP service will advertise as the maximum message size this Simple Mail Transfer Protocol (SMTP) virtual server will accept. If a mail client sends a message that exceeds the limit, they will get an error. If a remote server supports EHLO, it will detect the advertised maximum message size value when it connects to the SMTP virtual server and won't attempt to deliver a message that exceeds the limit. Instead it will simply send a non-delivery report (NDR) to the sender of the message. A remote server that does not support EHLO will try to send a message that exceeds the size limit, but will still end up sending an NDR to the sender when the message doesn't go through.
Limit session size to is the maximum amount of data accepted during the total connection. This is the sum of all messages sent during the connection (applying to the message body only).
You must be a member of the Administrators group on the local computer to perform the following procedure or procedures. As a security best practice, log on to your computer by using an account that is not in the Administrators group, and then use the runas command to run IIS Manager as an administrator. At a command prompt, type runas /User:Administrative_AccountName "mmc systemroot\system32\inetsrv\iis.msc".
To set message size limits