Test emails work, Gitlab emails don't (gmail smtp in v10.4.3)

I’ve found lots of v9 reports that were fixed by upgrading, but no solid thread that explains what might be wrong or how to approach debugging this problem.

Notify.test_email(‘user@example.com’, ‘Message Subject’, ‘Message Body’).deliver_now

works great from rails console… not gitlab triggered events (like registration or adding an email address to an account) work though…

How can we troubleshoot this and solve it? Thanks!