You're welcome,
But you can run a whole different OS version with Mail from an external drive.
The highest you can run onthat Mac is...
macOS Big Sur - Technical Specifications
General Requirements
OS X 10.9 or later
4GB of memory
35.5GB available storage on macOS Sierra or later*
Some features require an Apple ID; terms apply.
Some features require a compatible internet service provider; fees may apply.
Mac Hardware Requirements
For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Big Sur:
MacBook (2015 or later)
MacBook Air (2013 or later)
MacBook Pro (Late 2013 or later)
Mac mini (2014 or later)
iMac (2014 or later)
iMac Pro (2017 or later)
Mac Pro (2013 or later)
Learn about compatibility and how to identify your specific Mac model
https://apps.apple.com/us/app/macos-big-sur/id1526878132?ls=1&mt=12
And caution on 10.15 or higher, to see which Apps will no longer work in 10.15 get Go64...
https://www.stclairsoft.com/Go64/
For instance drives...
https://eshop.macsales.com/shop/external-drives/owc-ministack
https://eshop.macsales.com/shop/owc-envoy-pro-elektron
https://eshop.macsales.com/shop/firewire/1394/USB/EliteAL/eSATA_FW800_FW400_USB