There have been several mailing lists related to Allegro in the past. As of 2015, only the developer's mailing list is really used.
For general discussion you are encouraged to use the Allegro.cc forums.
[AD] Allegro Developers (firstname.lastname@example.org)
This list is for the people currently working on Allegro. You can use this address if you need to contact the developers, for example to submit some new code that you have written or to report a bug.
You can also browse older archives on SourceForge.
[AL] Allegro (email@example.com)
This list is for any kind of discussion about Allegro. It has very low traffic these days so you are more likely to receive a response on the forums.
[AGP] Allegro Game Programming
Between September 1998 and January 2003 existed the Allegro Game Programming mailing list. This list was used by people to ask about generic game programming problems not so closely related to Allegro. The mailing list dropped in popularity and was not moved to SourceForge, leaving the main Allegro mailing for both Allegro and generic programming problems.
[Alleg5] Allegro 5
This list was once available for discussing the next major version of Allegro, when it was too noisy to do so on [AD].
[Commits] Allegro Commits
This was a read only notification mailing list for commits to the Allegro repository. Nowadays you should watch for activity on GitHub.