Does this look like gmail is cracking down on piracy? Extra funny this is just them sending it to their kindle.
Also, Anna’s Archive linked me to a site for getting around this for Kindle but I forget + bookmarked on my laptop. Will edit this if I find it
It looks like a very basic screening tool to block suspicious file extensions from being uploaded/downloaded. I wouldn’t read (pun intended) into it too much, but it does mean you might have to work around it via sending epubs as zips or something. I doubt they bother scanning the contents of archives.