My nginx error.log shows the following:
*5 upstream sent invalid response: "* CAPABILITY IMAP4rev1 UNSELECT IDLE NAMESPACE QUOTA ID XLIST CHILDREN X-GM-EXT-1 UIDPLUS COMPRESS=DEFLATE ENABLE MOVE CONDSTORE ESEARCH
a1 OK test@example.com Test User authenticated (Success)" while reading response from upstream,...

It appears to not like google's CAPABILITY line. Is it too long? Any suggestions?