The error message isn't only about SimpleLogin. It lists Proton as blocked first. It doesn't make sense for an error to explicitly prohibit their own "real" domains. Even if it failed with an email on their domain, they wouldn't have written an error that specifically intentionally excludes their domain.

When quoting me, you chose to skip a whole sentence in the middle.

One that’s at least in part absolutely false.

The Proton domain is that absolutely false part.

The proper formatting when omitting a section of a quote is to replace it with something like this [...] The brackets denote that what's between is not literally the quoted words, but is an edit of some kind. Usually for clarity, or in this case brevity. Without them it looks like you're being dishonest in your quotation by making invisible edits. So it should have looked like this.

Truthfully that’s a very strange error message. [...] So much so, that I’m doubting the credibility of this.