Assuming that the Outlook credentials are stored and you are set up for Outlook Autology, WIN32OLE does the trick pretty nicely:
require 'win32ole'
outlook = WIN32OLE.new('Outlook.Application')
message = outlook.CreateItem(0)
message.Subject = "Hey look a subject!"
message.Body = "Yes this is dog"
message.Recipients.Add 'dog@dog.com'
message.Recipients.Add 'cat@dog.com'
message.Attachments.Add('C:\Path\To\File.txt')
message.Save
message.Send
" Outlook Ruby: ", .
, , , Outlook.