Well I know at one time IPS "was" developing a mail client you can get some information from this wiki article.... that was removed... so here is the wiki history of it:
Vectura is a [[webmail]] system designed to look and feel more like an actual desktop mail application.
A beta was originally promised in September 2005 <ref>"There is no set release date of course but definitely before the end of 2005 with the beta series in the next month or so." - Charles Warner, September 21 2005 -
http://forums.invisionpower.com/index.php?...416</ref>In February 2006, the link to Vectura on its main site and the discussion for it on the community forum was removed. Some customers speculated that it might go into public beta while others thought that they have stopped development on it.
Posts by the developer assigned the Vectura project in December 2006 mentioned that he has been assigned to other projects and that he will resume work once those projects are finished.<ref>"It is definitely something I want to pursue in the future and I still plan it and so on. But as Charles says, I have a lot of other work to do at IPS since I'm the only designer-type person around at the moment, which doesn't leave me any free time to work on Vectura unfortunately. So, hope to see it in the future, but I can't give a timeframe for that right now I'm afraid." - Rikki Tissier, Dec 10 2006 -
http://forums.invisionpower.com/index.php?...gt;<ref>I don't have a timeframe as such, because as I said in my previous post, it's all IPS tasks right now and that's going to be the case for a while yet. I do hope to do it soon though, because I enjoyed working on what I had done so far. - Rikki Tissier, Dec 11 2006 -
http://forums.invisionpower.com/index.php?...703</ref>So basically you want an addon that allows a user to signup for an e-mail @yourdomain.com while registering for Nexus... and then that e-mail gets imported into converge and is used for everything else on your site. The only thing is I thought only admins create accounts directly for Nexus, unless they are imported from IPB through Converge or another application added to converge.