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Often Gmail imposes certain limits on the number of emails sent within a time frame. The 450 error may occur if you exceed these limits or if Gmail detects unusual activity.
1. So first of all, if you are sending emails in bulk, you need to reduce Email Sending Volume. Now wait for 24 hours before resuming bulk email sending.
2. Let’s review Sent Emails, Check your Sent folder for excessive outgoing messages.
3. Sometimes, users may need to verify the google captcha. so you need to Visit Google’s Unlock Captcha page and follow the instructions to verify your account.
After completing these steps, try sending the email again.