I have just setup a new instance and seem to be getting this problem, but I think its a little deeper then we think.
This thread on Moodle forums talks about it and others that are having the problem.
I have my hands full on setting this new server up so will keep an eye on this thread to see how others go and if there is any fix.
As a note: it is the new Bitnami\AWS Moodle 3.3.2 that I am running with trying to config a google apps domain.
I have the settings on the google domain working as I get emails from the same setup but a moodle 3.1 setup.