So far Nemo Mobile is lacking an e-mail client, and Harmattan's one is closed source. Are there open and functional e-mail clients around which can be ported for handsets or tablets?

You can post your ideas here if you want. I think it's an important and "basic" usability feature (of course the client is far from basic, but I mean that it's something that user will naturally expect to be present on any mature system).

I spoke with Romaxa, and he said it should be possible to use Thunderbird engine with IPC embedding, if some front end UI would be written for example in Qt. It looks like a decent project to start, but it'll require quite some work.