I have two valid email addresses in my account. As of 5-9 hours ago, I tried to log into one of the SE sites (superuser), and couldn't get in. Long story short, I reset my password and still couldn't log in. I've now verified that I am able to log in with one of my two email addresses, but not the other, in spite of the fact that the password reset email told me I could use either. I verified this by copy-pasting the password and email address just to be extra-safe. In case it helps, the email address that worked was an old email address that I signed up for SE with, the other (that doesn't work) is the new email I actively use. The latter is the email address listed in my preferences on SE.
UPDATE I've verified this behavior on 3 computers (Windows 10 and 7) and 2 browsers (Chrome and Edge).