I think this plugin has a LOT of potential. However, it's

Hello, I think this plugin has a LOT of potential. However, it's currently "stuck". There haven't been any updates at all to it beyond January of this year.

I will give back to the community that has given such a pleasant surprise. You can find my fork of automessage here:
http://www.otleylogic.com/screenshots/automessage-pFm324GO5e.zip

The developer is free to take and integrate it with his own as he pleases. I do hope some of my features make it back into WPMU's version. Some major additions are:

1) Support for HTML email

2) Support for custom hooks (instead of just wpmu_new_blog and user_registered).

3) Automessages are now broken into two levels. First is blog-level. These automessages can be set by blogs individually. Above that, is global-level. If an automessage is not found in blog-level, it looks for it in global-level. Global-level automessages are set from the MAIN blog only.

An example of the new interface:
http://www.otleylogic.com/screenshots/Screen%20Shot%202013-10-24%20at%2012.39.02%20AM-cLeP6A23lk.png

Thank you, let me know if you have questions on it. The entire queueing system had to be re-written to handle multiple hooks.