Mozilla Messaging (abbreviated MoMo and formerly known as MailCo) was a wholly owned, for-profit subsidiary of the non-profit Mozilla Foundation. It was broadly tasked with aspects of the Mozilla Project that focused on interpersonal communications, such as instant messaging and e-mail. Its main focus was developing version 3.0 of Mozilla Thunderbird, the e-mail client developed by the Mozilla Foundation.

It was spun off from the Mozilla project in 2007; on 4 April 2011, it was merged into the Mozilla Labs group of the Mozilla Corporation.