The IRecipientSource interface can be implemented to fetch recipients from essentially any data source depending on what code is used to implement the methods to fetch the recipients.

An IRecipientSource implementation can present one or multiple recipient containers (essentially recipient lists) which are grouped by site (aka section in EPiMail).

The definition of the recipientcontainers themselves can be hardcoded in the IRecipientSource implementation in the simplest scenario, or be loaded dynimcally somehow depending on requirements.

The main requirement is essentially that you can get the email address and a GUID for each recipient, so that you can create the recipient instances as necessary.

Note: If you will have large number of recipients that are to be fetched from a lock congested table it may be necessary to create some type of prepopulated "cache" table that can be repopulated during low-traffic hours or similar.

I've actually written a blog post about a Custom Recipient Source for EPiServer Mail 4, but the server is... missing. I'll try to find some time to write a new post, but I have the source here. Use "as is" and be sure to backup 'n stuff.

http://viktor.likeablestudios.com/recipientsource.zip