450, "4.2.1", The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered. Please resend your message at a later time. If the user is able to receive mail at that time, your message will be delivered. For more information, see Limits for sending & getting mail.
And here's what it means.
First, understand that it does NOT mean that your sending IP address and sending domain are blocked from sending mail to Gmail. This is not the same as being blocked for having a poor sending reputation!
What it means is that the RECIPIENT EMAIL ADDRESSES that you are sending to are receiving too much mail, much too fast. There are a few different scenarios that can cause this.
First, are you using your own personal or test Gmail accounts to send many messages to for "load" or "stress" testing? Don't do this! Sending too much mail (they never say exactly how much is too much) to your own test addresses will cause this error message to be returned. And it can back up your own sending email queue, delaying delivery of legit mail to legit subscribers.
Any time you're going to do load testing or stress testing sends from an email platform, make sure you send ONLY to mailboxes you control at servers you control. Not mailboxes hosted by Gmail, Microsoft or some other ISP. Not mailboxes behind spam filtering like Proofpoint or Vade Secure. Trying to squirt a whole bunch of mail to a small number of test addresses is not a use case that most spam filterers or ISPs know how to account for. This happens somewhat often. I can't stress enough -- email stress testing is not compatible with ISP or service provider-hosted mailboxes.
This also can happen sometimes even at lower volumes, even if you're doing something like sending one email per minute to your test Gmail account to make sure a system is alive. Testing daily or perhaps even hourly is probably fine, but at some point if your interval is too small, you're going to have problems, and this is a primary way that those problems will manifest.
Next, is it possible that somebody is submitting a bunch of junk addresses to your subscriber list signup process and now those addresses are getting too much mail (maybe even overall, not just from you). If you are utilizing "lead gen" or "co-reg" email address signup processes, this can happen. It's a sign that your signup partner might be selling those same addresses to more senders than they've told you about. If one signup on one form results in a hundred email followups from a hundred different senders, you could end up with this. Payday loan senders are notorious for this, but others engage in it as well. Or, it could simply be some form of bot activity-- if it's not any sort of third-party related process, it could be as simple as fixing it by adding a CAPTCHA or double opt-in to the signup process.
Whoops, did you accidentally "mail bomb" a bunch of email recipients? Could it be that you accidentally launched an email campaign to the same subscriber list a bunch of times (10 times, 50 times, 5000 times) and your mail server is dutifully trying to deliver all those messages? It happens. But when it happens, see if you can work with your sending platform or your IT team (if it's an internal system) to delete pending email messages from the mail queue to clean it up.
The key here is whatever you're doing, whoever you're sending to, stop doing that. Figure out which process or practice or partner is the source of pain, and address it. That's what stops this from recurring.
For future reference, here is a link to Google's overview of the different Gmail SMTP error response codes.