While some people used Tormail to email clearnet recipients (and could use many clearnet email providers over Tor to do the same), the majority of Tormail users used it as an internal messaging system. Something as simple as TorPM could be an effective replacement, as long as it is stable and easily accessible (bonus if you can add attachments to messages). The down side is that you have to trust the operator of a centralized system to not read your messages or look at the metadata of who you are communicating with if you use PGP, but a messaging system like TorPM is easy to set up. I'm surprised a replacement hasn't come along yet.