Reliably Sending Emails Through Gmail SMTP

I have a C# application that sends emails through smtp.gmail.com. If I ever send about 20 emails, I start getting errors like

The server response was: 4.7.0 Temporary System Problem.  Try again later (WS)

Usually, about 17 messages out of 20 ever end up getting sent.

Note that I am not saying 20 messages were sent and only 17 were successfully delivered, I mean that only 17 actually even try to get sent.

I can't find much specific info on this error, but I assume this is relat开发者_如何学JAVAed to some kind of a gmail throttling limit, because I don't see this error if I only send like 1-5 emails at a time.

Can someone confirm what this means and why it happens?