"transient error (421): 4.7.0" when forwarding emails to Gmail

My emails were set to forwarding to a Gmail. I was working fine. But recently, most emails were rejected by Google. In the Activity Log, I saw errors like this:

Unknown error: transient error (421): 4.7.0 [ 19] Gmail has detected that this message is4.7.0 suspicious due to the very low reputation of the sending domain. To4.7.0 best protect our users from spam, the message has been blocked. For4.7.0 more information, go to4.7.0 Why has Gmail blocked my messages? - Gmail Help w8-20020a05622a190800b0042edebced5bsi9021171qtc.485 - gsmtp


Same here. The elevated errors started last Thursday and exacerbated over the weekend.

Looks like gmail blocklisted a large block of CF sender IPs.


Let’s escalate this to google: Sender Contact Form - Gmail Help

In this case, CF is the bulk sender for forwarding emails to gmail.

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I’m also getting this problem - is this for Cloudflare to pick up with Gmail?


It is for you to pick up with Gmail.

You’re the one selecting Gmail, as the final destination, for your email traffic.

Gmail is refusing messages, which you actually want to see, so therefore, you need to send your complaint to Gmail.


I’ve raised a complaint with Gmail however my interpretation is that Cloudflare are the bulk sender so responsible for this?

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I think Cloudflare may need to discuss with Google on this matter.


Anyone having some alternative solutions other than Cloudflare?

I was just looking at forwardemail.net


Forwarding was an unreliable delivery method long before Cloudflare introduced their Email Routing service. You have no control of the email authentication practices of the sender and when you forward to a mailserver that you cannot configure, such as one of the freemail providers, you are at the mercy of their policies. When they defer a source, you cannot override that. You can only wait it out or change to a better delivery system. That may be a lower volume forwarding service, but ideally would be switching to MX routing.

I have advised against forwarding to freemail providers for over a decade and I stand by that recommendation in all cases where reliably receiving email is important.

It doesn’t mean that forwarding has no purpose, but it is not a good option for an address that delivers to a personal mailbox, especially on a freemail provider. Adding a domain email address as an alias on a paid mailbox will always be more dependable. There is no shortage of low cost options available for those who are not lookingto break the bank. Free options exist, too, but may take more effort to match specific needs.

I already gave this a try and it is the same issue. Gmail also blocks mails and the worst is, that you get no log or something like that from forwardemail.net

This is what I did: Cloudflare Email Routing to Gmail


I understand routing email directly into Gmail would be a better option but this, to my knowledge, requires Google Workspace, which is a paid service.

As Cloudflare are providing a service to route emails, in my option there should be some warnings of unreliability and options to better handle failures.

We could look at alternatives but that is avoiding rather than solving the current issue. However, I understand you’re saying there is no solution and no one should use Email Routing to free email providers (GMail, Hotmail, etc.).

Same here. :confused:

A post was split to a new topic: Email routing questions 2

The writing was on the wall back in the early twenty teens that forwarding was going to become a lot less useful as the arms race with spammers continued to escalate. The widespread adoption of DMARC has fulfilled that prediction.

While not the only reliable option, oftentimes a paid solution is the best solution, especially if the email is important. There are more options than Google and Microsoft for email, though they are certainly the most visible.

For those who want to use domain email with Gmail and not pay for Google Workspace, my recommendation is to configure direct to MX on another provider. Bundled email from bulk hosting is sufficient and storage capacity largely irrelevant once the account is added to a Gmail account. Gmail can then be configured to regularly sweep the mailbox contents into Gmail using POP3 with SSL. One major upside of that approach is that the external email account typically has an SMTP server that can be used with Gmail to send DMARC compliant email.

My major issue with Gmail POP3 was it didn’t sync in real time, which was frustrating using things like two factor authentication / sign in codes. I currently run external emails fully authenticated through Amazon Simple Email Service, which works well.

I do agree with your points and maybe it is time to put my hand in my pocket :slight_smile:


That is but one reason I prefer TOTP or a Yubikey for 2FA. Sadly that isn’t an option with one of my vendors who only offers sign-in via emailed codes, so I am appreciative of that limitation of the Gmail pull approach.

I don’t know your comfort level or how hands-on you want to be with your email management, but there are a lot of options that range from buying a mailbox to running a virtual mail platform like MXRoute. The latter is definitely more involved than simply subscribing to a mailbox, but can offer a happy medium between just a mailbox and running your own mailserver (which is not something to be taken lightly).

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We’d just created a dot engineering TLD domain for a client of ours. We’d forwarded the emails using Cloudflare, got the error described in the beginning and Google Search brought us here.

The first two emails failed to be delivered initially. But the second email landed in recipient’s gmail spam. We marked not spam. Then we tried a few emails back and forth and all went through. Perhaps some changes happened very recently?

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Things change constantly as spam ebbs and flows. That is part of what makes forwarding email unreliable. It can work fine one moment and stop for days before suddenly working again.


To add my resolution. I’ve taken the plunge and moved to Amazon WorkMail to set up a proper mailbox rather than forwarding to Gmail. Thanks for your help and advice.

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