After upgrading to 2.3.1, registration emails don't arrive at many addresses. Downgrading to 2.2 allows emails to the same addresses to go through.
I then did a fresh install of 2.3.1 (not an upgrade), and the same issue occurs. The registration emails via 2.3.1 will get through to less than 1/3 of the email addresses I try (yes, I've checked spam folders, and reset one address so that no spam checking occurs!). Those same addresses work fine in 2.2
Is there a difference in the way 2.3.1 handles sending out emails that might cause this difficulty?
(With the fresh install, there were no plugins, the default theme was used, etc.)