I got following logs.
{P1921}

Env

• nginx version: nginx/1.6.2
• PHP 5.6.14-0+deb8u1 (cli) (built: Oct 4 2015 16:13:10)
• latest phabricator.
• Use gmail smtp as mail sender.

I don't know It appear on secure.phabricator.com or not.
And, I don't know when was this issue first appeared either.
So, I can't let you know the reproduce step. sorry.

The log contain mail-reply handler related function name. but I don't have any mail send problem.

I don't care this issue is supported or not. (cuz, there is no explicit issue using phabricator.), But just report error logs.

PS. the error log appear periodically. (about 5 min)

Sorry, my poor english.