I discovered this issue while building a site with Membership 2 Pro and had thought my upgrade to Membership 2 Pro had caused the issue; but am now wondering if it's related to the recent update of WordPress. I say that, because I've noticed a lot of WP users have had this same issue.
The site had been sending new user registration emails (username & password reset) without a problem, then suddenly stopped.
I do get an admin notification email of a new user.
Membership 2 Pro also sends an automated "Signup Completed with Payment" (and "Free Membership") email.
So the site IS sending emails, just not new user registration emails.
I noticed some people suggested trying the following plugins.
I've tried WP-MAIL-SMTP plugin. It didn't help.
I've tried Easy WP SMTP plugin. It didn't help.
Any insights would be greatly appreciated.