I don't have answers but here is my thread on the same problem.
https://www.mobileread.com/forums/sh...d.php?t=332177
Now I have ONE answer--yeah, it's not just me. When I use Send to Kindle on my browser, it uses my gmail account and that does not generate a verification from Amazon. I don't want to use my gmail account for Calibre for security reasons as it's my primary email address. I created a GMX specifically for Calibre. I tried to create a hotmail account but it redirects to Outlook. I created an account anyway for Calibre but the Calibre test was unsuccessful. I thought about creating a gmail account solely for Calibre but I understand gmail doesn't play nicely with Calibre?
At this point, I'm resigned to verifying.