Configuring the Maximum email message size

Configure->LocalConfig->MaxMailSize

This setting determines the maximum size of any single email that the Server will accept either from a local computer or from the Internet. The default value is 1MByte, which represents some 3-5 minutes of on-line time when using a typical dial-up connection.

You can set this vaue between 500Kbytes and 50Mbytes- which could take several hours to deliver even on a reliable link.  If you are sending large emails, you should also consider delivering them via an external mail relay.