Hi Dear,

That is a huge amount of data to export from Office 365 mailboxes. You can do a couple of things to download your emails. 

#1. Configure Your O365 with Outlook Desktop Application: Follow the below instructions to export Office 365 emails to your desktop. First, make sure to install the any desktop version of MS Outlook.

1. Go to your system’s Control Panel and choose Mail
2. Click on Add option on the Mail window.
3. Now in the opened dialog box choose Manual setup or additional server types >> hit Next.
4. Here choose “Office 365”, followed by entering your Office 365 login credentials >> click Next
5. Your Office 365 account will be verified in a while and then you will see Autodiscover.xml login window where you need to enter O365 login details >> hit Next.
6. If everything goes right, you will receive a notification window with Congratulation messages.

You have to open your Outlook account and check all your Office 365 emails.
Now that you have configured Office 365 account with Outlook desktop version, it’s time to download Office 365 emails to desktop. Follow the below steps:

1. Open Outlook >> go to File option >> click on Import & Export
2. Under Import & export wizard choose Export to a File
3. Choose Outlook data file option to migrate emails to PST file.
4. Mark check the checkbox to include Subfolders >> click Finish.

Reference from: https://www.systoolsgroup.com/how-to/download-all-office-365-online-emails/​​​​​​​ (https://www.systoolsgroup.com/how-to/download-all-office-365-online-emails/" target="_blank)

#2. Use Office 365 Backup Tool: Out of all the ways to download Office 365 emails, the most convenient and safe method is Office 365 Backup tool. The software is quick and super secure to export O365 data to desktop. Since the data is huge in your case, I believe this backup software is ideal. You can get the software here: https://www.systoolsgroup.com/office365-backup/ (https://www.systoolsgroup.com/office365-backup/" target="_blank)

#3. PowerShell Commands: As manual solution, PowerShell comdlt can also be used. But it is really complex and lengthy, also if the data is huge, this method is not ideal. 
You can get the implementation of PowerShell method here: https://windowsinstructed.com/how-to-backup-microsoft-exchange-online-mailbox-to-pst/ (https://windowsinstructed.com/how-to-backup-microsoft-exchange-online-mailbox-to-pst/" target="_blank)

Hope this is helpful.

Thanks & Regards,
Andrew