When you submit a password that isn't secure enough, it should mention that you need special characters, number, letters, etc. Also if it requires it to not contain words in the dictionary, please specify. I had to dig up an old Mailgun blog post after reaching out to support, and the password criteria they listed still didn't work. I found out by trial and error that random gibberish worked but something with a word in it did not. Since there was no listed criteria, all of this was a mystery to me