Google Gmail SMTP; Read Error: Generic::Failed_Precondition: Read Error (0)

When trying to send an email to an address from a Google GMail server (whether it’s the free or business service with your own domain), you get this error:

Action: delayed
Status: 4.4.2
Diagnostic-Code: smtp; read error: generic::failed_precondition: read error (0): error

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.