Responding to a comment on the PR:

> Now, that said you might want to consider the fact that in _fold_mime_parameters I deal with this issue by bumping maxlen to 78 rather than raising an error. I'm not sure that was the right choice, but whatever we do, it should probably be made consistent between the two cases.

So I think in an ideal world this would be consistent between the two, but I *also* think that the right solution is to raise an exception rather than silently coercing, and changing this in _fold_mime_parameters would be a backwards-incompatible change.

I think that if you agree that an exception is better, maybe the path forward is to raise an exception in this case and switch the _fold_mime_parameters case over to raising a warning, which will be turned into an exception in a later release (3.10 maybe).

> It is so sad that I never came back to fix that XXX comment I left myself :(

I'm glad that the XXX comment was at least there! Otherwise I wouldn't have notice that there was anything to fix!