I've correctly configured all of my domains for Platform Email, and enabled it on my environment, but emails aren't sending.
It could be that the From email address is not on the list of approved domains. See Platform Email: From Addresses in Outbound Email Must Be From Approved Domains.
Platform email does not allow the spoofing of site visitor emails, as we strictly enforce what is a verified sender of email. There are workarounds to this, especially in the Webform module as described in the link above. If you've done that, file a ticket with Acquia Support and we can take a closer look.