Any ideas on how you managed to resolve this? I am facing a similar issue
Looking at the celery logs, I can see Celery reporting a successful send, but I can't see any log of the mail going out on the MTA (I'm using Sendmail not Postfix). I am guessing that the job in the celery queue is attempting to login to the server running on port 25? But I can't see any log of this interaction anywhere. I have a external SMTP relay server - but I am not sure which settings to change in the public.yml file to enable this. Do you know if this is possible?
Using Sendmail isn't supported by CommCare. This is mostly because Django doesn't support it directly (#8961 (Add support for sending emails directly through MTA) – Django). It should be possible to get it working but it would require some manual steps and probably some changes to commcare-cloud to support installing additional libraries during deploy.