Microsoft allow you to get Outlook.com mail with your custom domain [1], and the free version includes up to 50 free users for a domain. The Outlook.com web interface is not bad at all, and you should be able to access your mail over pop3 (EDIT: no imap). To use it you just need to add a bunch of custom DNS records to your domain [2], exactly like you used to do with Google.
Another option is Zoho, which allow 3 users in their free plans [3]. As a bonus you also get access to their office suite.
[1] https://domains.live.com [2] http://www.labnol.org/internet/setup-outlook-on-custom-domai... [3] http://domains.zoho.com/
[1] http://gadgets.ndtv.com/internet/news/microsoft-says-outlook...