Baku is a simple SMTP server written in ruby. Fire'd up it will accept N number of emails from whatever MTA you're using and then store them in Redis.
Baku is EXTEREMLY fast handling ~2.3k emails a second and is useful for load testing applications that generate lots of mail.
Shipping with Baku is a sinatra web-ui that enables users to view emails stored by Baku.
Additionally, Baku comes with Tsurimi, a stripped down, extremely basic Imap implementation in ruby that allows users to programmatically access the messages store in Redis.
These tools are designed to facilitate both load and functional testing of ruby apps that generate email. With Baku running your ruby app can generate tens of thousands of emails and have them quickly stored both for manual testing (using the sinatra app) and automated testing through rspec, cucumber and friends through the imap interface.