What Does This Error Mean?
A 504 error is a gateway timeout, meaning Smartlead attempted to connect to Google's servers but did not receive a response in time. In the context of Gmail accounts, this typically happens when Google has flagged the account for suspicious activity or the account is being rate-limited, causing the connection to time out rather than complete normally.
How to Fix It
Work through the steps below in order. Stop as soon as the error is resolved.
Step 1: Make sure you are signed in to the correct Google account
Before anything else, confirm you are signed in to the specific Google account that is showing the error in Smartlead. If you manage multiple accounts, sign out of all others and sign in to only the affected one.
Step 2: Visit the DisplayUnlockCaptcha page
While signed in to the affected account, go to: https://accounts.google.com/b/0/DisplayUnlockCaptcha
Click Continue. This tells Google that the recent access attempts from third-party apps like Smartlead are authorised by you.
Step 2a: If the DisplayUnlockCaptcha page is unavailable
If the page does not load or shows as unavailable, skip it and move to Step 3.
Step 3: Refresh your app password
If the error persists after Step 2, refresh your authentication credentials:
In your Google account, go to Security settings
Turn 2-Step Verification off, then turn it back on to reset the settings
Delete any existing app passwords previously created for Smartlead
Create a new app password for Smartlead and copy it
Go back to Smartlead, open Email Accounts, find the affected account, and paste the new app password into the credentials field
Click Verify your account to confirm the update
Step 4: Reduce your daily sending limit
In Smartlead, go to Email Accounts and open the affected account
In the general settings, reduce Messages Per Day to 40 or lower
Click Save
If you see an error when saving, wait 24 hours and try again.
Why Does This Happen?
This error can occur when a Gmail account is being pushed beyond its comfortable sending thresholds. Gmail accounts are primarily designed for personal and business communication, not high-volume outbound campaigns. If you are regularly hitting this error, consider:
Warming up the account by sending no more than 30 emails per day for 1 to 2 weeks before scaling up
Spreading your sending volume across multiple inboxes rather than concentrating it on one account
Still Seeing the Error?
If the issue continues after completing all steps above, contact Smartlead support with the affected email address and a description of where in the process you are getting stuck.