From Google Takeout - Wikipedia
On December 5, 2013, Google Takeout was further expanded to include
Gmail and Google Calendar data.
Formerly named Google Takeout, "Google Download your data" offer Gmail messages in MBOX format.
From Download your data - Accounts Help
You can download the data associated with your Google Account so that
you can use it in another service or keep a copy for your records.
You can request this info using the “Download your Data” tool. Here’s
Start your download
- Sign in to My Account.
- In the "Personal Info & Privacy" section, select Control your content.
- In the "Download your Data" box, select Create Archive.
- Select the Google products you’d like to include in your download and select Next.
- Choose the file type that you’d like your data in and how you want to get it (either by a download link or directly into your Google
- Select Create archive.
Once the archive is created, you’ll get an email to let you know it’s
ready. Depending on the amount of information in your account, this
process could take a few minutes or several hours, but most people get
their link the same day they request it.