I don't know why e-Newsletter should play with the rewrite_rules, but it did break my website.
It took me a whole 24+ hours to find out why my website wasn't working anymore.
Eventually, in a test environment, as soon as I've disabled e-Newsletter, the website started working again.
What happened exactly?
I've a two language website. The default language is Italian, the other one is French.
The website relies on WPML.
translated contents are handled using rewriting rules for permalinks.
Where an URL in italian may look like domain.tld/something, the french translation looks like domain.tld/fr/something
Well, the french contents returned a 404 page each time. Not just that. When this happened, also the italian contents were showing a 404 error page.
Resaving permalinks made me get back the Italian contents, but as soon as someone (me or anyone else) asked for French contents, I was getting again 404 error pages.
This happened with custom post types only: pages and posts were still working.
The WPML developers have been informed already (they helped me a lot to sort out the issue), but I think that also the e-Newsletter developer need to take a look to his code: this is the only plugin (among 63 installed and enabled) that causes this issue. Other plugin clearly works with permalinks, rewriting rules, etc, but doesn't cause this issue.
e-Newsletter, instead, looks is resetting this rules, even when he's not even called: the issue happens in pages where e-Newsletter has nothing to do at all.