While the diagnostic code is useless (smtp; read error: generic::failed_precondition: read error (0): error is actually just a generic fail message with no real information in there), the status code is useful.
The status 4.4.2, is actually an SMTP status error code, and translates to:
The connection was dropped during transmission
Your server started delivering the message but the connection was broken during transmission. This may be an unusual transient error – however, if it keeps happening you should investigate possible problems with your server’s network card, your Internet routers, processes hogging the resources of your server, and anything else which could result in a network connection being broken.
This tells you a lot about the problem. It means that Google’s server lost the connection when it was trying to send the email.