Enable Developer Mode (https://www.odoo.com/documentation/19.0/applications/general/developer_mode.html) and navigate to Settings -> Technical --> Email -> Emails. In this list, find the message you've tried to send and check its state.

State Sent suggests that the outgoing server has done its duty but the receiving server potentially hasn't acknowledged that mail. This could mean a spam protection measure on the receiving server - or that the mail simply hasn't been internally delivered to the correct inbox yet.

State Received suggests that the receiving server actually received the mail. It may still be directed to a Spam destination.

Delivery Failed has a multitude of reasons - check the Failure Reason-tab in the message.

This would be crucial to figure where things stop working. 

Depending on the outcome, some options may be:

• First I would create a completely new integration to ensure no old data remains in the Auth process
• Assuming we're talking about an actual GSuite account, https://admin.google.com allows you to investigate in more detail using Reporting -> Email Log Search for both, incoming and outgoing mails
• If your end looks good and the message actually goes through (according to the logs), send it to a server you have access to logs to or have someone you can check their end as well to figure out what might be wrong
• Tools such as https://mxtoolbox.com can help you identify issues with your mailing domain if there is any chance of this being the breaking point

Unless you have a Delivery Failed reason, in most cases this has to do with some sort of spam protection.