The Piazza and the Mystara Reborn group on Facebook.
WotC announced that they will be shutting down all of their mailinglists on February 15th:
On February 15, 2015, Wizards of the Coast will terminate our LISTSERV
system that manages this (and other) mailing lists. We've watched the usage
of these mailing lists dwindle due to the rise of other communication
platforms, such as forums and social media. Given this decreased usage, we
have made the decision that it was time to turn them off.
Please take these two weeks to back up or archive any emails you wish to
keep. After February 15, 2015, they will no longer be available and these
lists will cease to function.
—Wizards of the Coast
It is a god thing that they are giving us at least a few weeks notice before the lists are shut down. Although there hasn't been much activity on the list in recent years, it is disappointing that the list archive is also being deleted. Fans are currently discussing options for preserving the archived email exchanges on other websites.
In any case, this marks the end of an era. The MML was the arena where many friendships were formed. It was also a list where TSR alumni such as Bruce Heard, Frank Mentzer and Harold Johnson participated. Fan undertakings such as the Mystara Net Almanacs, the Tome of Mystara, the M3E Project and many other projects were initiated.