This is duplicate of now closed T11130 with more information. I thought I gave enough details talking about rule I created and output of the message I saw. Anyway as more information was expected in a new bug, I am adding this one.
I have created an herald rule, screenshot of which is below.
The message tab shows the following, where To is None and Cc is vivekanand (me)
And finally, this is what the status shows. "invalid address: null"
That is all I can see in the mail metadata tab, and mail is not sent, delivery is failed.
Now just to be sure the empty metamta.placeholder-to-recipient is causing the problem, I added an email to it and resent (using ./bin/mail resend) the mail, it works perfectly, the mail is sent.
Just to be sure, this is how the config screen looks when the mail fails to deliver.
And the versions are : -
phabricator 77d543ba44638a2e43e1b6bb7f75db80b4046409 (Sat, May 21)
arcanist 0249f90a9a4aa42da0823673a6ac8c620251d82a (Sat, May 21)
phutil 7ff4bd8dfb0b58686a72821dd0e9e79e0f5cbbb9 (Sat, May 21)
Local Version Bitnami Phabricator 20160523-0
Hope the information given here is enough and helpful. I am ready to add more information if needed.