Can you clarify what you mean by "got a link"? If you're using Send to Email, there shouldn't be any links involved. You just, well, send an email to the designated address with the ePub attached, and it shows up in your library shortly after.

Amazon asks you to verify the upload was intended, by sending a link you need to click. It's been like that for years.

FWIW I have been using that feature for well over a decade, with several hundred third party books in my Kindle library, and I haven't seen what you described once. Is the sender on the pre-approved list in Send to Email settings?

I've been sending files to my Kindle for years, I've never seen that.

The web app for sending to Kindle is a lot more reliable:

https://www.amazon.com/sendtokindle

you probably just didn't send it from a verified send-to-kindle email address. either add the email or send the epub from your kindle account email