The plugin adds images to emails by encoding, embedding and optionally compressing them. Compression will be used for pictures with a pixel size (width or height) greater or equal to the value labeled Maximum length of a side of a picture in pixels. Enter 0, if you don't want pictures to be compressed.
In order for the plugin to work, you have to have a Mail Server configured. Choose that Mail Server in the drop down labeled Default Mail Server for the plugin.
Mail Servers usually can't handle emails with thousands of recipients. How many recipients per mail are possible depends on the setup of the mail server. The recommended minimum from the RFC is 100. If you know that your mail server can handle more, you can enter your own limit into the recipients limit field. Users will be informed that their page will be sent out in individual emails if they try to send to groups larger than the limit set here.
Finally, you can decide, which icon will be displayed in the header of an email - your globally used Confluence icon (e.g. company logo) or the icon used in the Space the shared page belongs to (e.g. department logo)