https://github.com/PrestaShop/PrestaShop/issues/11933 I got this. Can't reproduce error. However, there might be something in this :
https://central.owncloud.org/t/problem-sending-the-email/15810
that might be related to it. Seems to be something to do with Red Hat, if I read correctly.
Let's update :
Did a complete new install from the same PS archive I downloaded a few days ago. It refuses to send mail too, so it's not a "migration" problem.