If you receive an SMTP 454 4.7.0 error under your Google account when attempting to connect it to the platform, this typically is an indication that one of the following reasons listed below is affecting your inbox.
1) You have too many concurrent connections/services synched with this email address (Note: Every time you connect an inbox to a service, whether it be a mail client, an online tool, or logging into a webmail client, this counts as a connection. Under Google, the limit is 10 concurrent connections and some services can utilize multiple concurrent connections)
2) You/someone else has attempted too many logins using an incorrect password within a short period of time
3) You have reached an email sending limit under your inbox and are temporary locked out of making any new connection attempts to this inbox.
4) You do not have DKIM setup properly and Google is getting hung up authenticating your inbox.
How Do I Resolve This Issue?
For most users of our service, the most likely cause for this error being thrown is having an inbox connected to multiple tools/email clients. The first thing you should do before attempting any other methods to resolve this issue is to track down where you have this specific inbox connected across the internet. To do this, please refer to Google's Help Desk article on checking 3rd party app connections found here. This will highlight all applications connected to your inbox to give you a better idea on if you are hitting a connection threshold.
One thing to note is that not all connected services will show up under Google's 3rd party app management section. If you have an inbox connected via SMTP & IMAP to an email client (such as Mozilla Thunderbird, or a inbox management app on your phone), this will still count as a connection to your inbox and would be adding to this connection limit, so double check that your inbox is not connected to anything that you are not using still.
For the various other reasons why this issue can arise, please try going through these troubleshooting steps below.
1) Make sure that your account is secure and no one is attempting to gain access to your inbox. To do this, navigate to the security section of your inbox under account settings and check to see if there have been any strange login attempts that you are not familiar with.
2) If you have hit the Google sending limit under this inbox (The limit is 2000 sends in 24 hours, but Google can lock you out/limit you further if you have had issues with spam complaints), you will need to wait 24 hours until your sending limit has reset and try connecting again.
3) If you do not have DKIM setup under a sender domain, this error can occasionally get thrown, so make sure that you have this security protocol properly in place under your domain if you have tried all other troubleshooting steps.