Import a Google Takeout archive into a Gmail account

Holding a Google Takeout of my old university account, the big MBOX file plus contacts and such and I want the mail inside my current personal Gmail.

Gmail seems to have no import a Takeout button. What actually works?

Correct observation, the button does not exist: Takeout is an exit door with no matching entrance, and the MBOX inside it needs a client to carry it back in. Thunderbird is the free carrier of choice.

1

In Thunderbird add your current Gmail account over IMAP, signing in through the modern OAuth prompt and let it connect.

2

Install the ImportExportTools NG add-on, then right click Local Folders, ImportExportTools NG, Import mbox file, selecting the Takeout's mbox. It arrives as a local folder holding the old account's mail with labels flattened, since Takeout exports one mbox with label information in headers rather than folders.

3

Create a folder under the Gmail account, OldUniversityMail or per your taste and drag the imported mail into it in batches of a few hundred, each batch uploading to Gmail's servers as it lands.

4

Verify in the Gmail web interface where the folder appears as a label, then the local copy in Thunderbird can stay as a bonus backup or go.

Two scale notes: a multi gigabyte mbox imports into Thunderbird faster than it uploads to Gmail, the upload being days for very large archives at IMAP pace, batched patiently. And the flattened labels are recoverable in principle, each message carries an X-Gmail-Labels header the determined can filter on after upload, though most people find the single archive label entirely sufficient for old university mail. Contacts from the Takeout import separately at contacts.google.com in two minutes.

4GB mbox imported locally in minutes and the upload ran two evenings in batches. One archive label suits the purpose exactly as predicted. University years preserved inside the account I actually use.