Open source software developers probably won't want to touch this with a bargepole. Oddball copyleft licenses are a pain in the ass to deal with an invariably incompatible with every other copyleft license including the GPL.

Came here to say much the same. GPL / non-MIT generally = avoid, because IANAL and don't want to figure out if I'm in the right or wrong.

Note that EUPL is explicitly compatible with GPL and a few other licenses in that it allows you to license a derivative work under GPL (or the other license) instead of EUPL.

The downside to this approach is that you need to get copyright attributions for anyone who submits code to the project. This is often a fairly large barrier to entry for contributors and I suspect that the author will continue to have poor engagement for outside contributions.

Now, from a practical point of view, that may be perfectly fine for him. If he's not getting many contributions anyway and doesn't really care about it, funding the project through license sales can make sense. Sometimes I think people don't think about the trade off that's required, though. I've seen people get frustrated with the lack of outside participation.

However, I agree with you. In this case, it seems like a smart move.

Yeah, you're right, I don't care about external contributions. I never wanted this to be a "community" thing. It started as I got interested in the quirks of email and wanted to mess around with the topic. Trying to find consensus etc. just makes it harder, not easier.

You can use Supported Source:

https://supportedsource.org/

It's not copyleft, but if you ask you might be able to use it that way.

Not only is it not copyleft, it's not free software nor open source software. It's proprietary software by another name.

Supported Source is more or less shareware with visible source code. It's not free software or open source.

Selling copyleft exceptions works fine with existing free and open source licenses.