Unfortunately we have not been able to fix this issue in the Phabricator Stack yet. However, I have taken a quick look into this issue and got it solved by executing a few commands.
In order to enter the Phabricator console:
Could you try to execute the following commands to check if it helps to solve the issue?
./config set metamta.can-send-as-user true
./config set metamta.reply-handler-domain YOURDOMAIN.com
Don't forget to restart your Phabricator servers afterwards:
sudo /opt/bitnami/ctlscript.sh restart
I hope it works!