Disable users from sending emails from old Gmail accounts after migration to Office 365

An organization completed a migration from Gmail to Office 365 email 9 months ago.

All incoming emails arrive in the user's new Office 365 email accounts and any time that corporate email accounts are set up these email accounts auto-configure with the Office 365 email settings.

This organization's public DNS records are being managed by a company named A Small Orange (http://www.asmallorange.com). At this time we are not able to switch to a different public DNS record company or use Office 365 for managing this organization's public DNS records.

The Gmail accounts were left active since the organization has continued to use Google Drive and won't be migrating from Google Drive to Box for another couple months. As soon as this Google Drive to Box migration is completed the Gmail corporate account will be completely deactivated.

The problem is that users are still able to login to the Gmail.com website with their old corporate email accounts (username@companydomainname.com) and send emails from the old Gmail email accounts and these emails arrive in the recipient's Junk email folders or sometimes within their Inboxes.

I need to make sure that these users are not able to send any emails from their old corporate Gmail email accounts while leaving these user's access to their Google Drive data intact.

How can this be done?
IT GuyNetwork EngineerAsked:
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Dmitri FarafontovLinux Systems AdminCommented:
Setup Google Cloud Platform (https://cloud.google.com), confirm your domain and manage policies from there.

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Dmitri FarafontovLinux Systems AdminCommented:
Thanks for the accept.
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