Aug 10, 2018 - 07:00 AM
The Gmail account is retrievable from the server at any time. About the On My Mac mails, you will need a good Mac Mail backup software. I have been using Mail Backup X for the same. Take a free trial of this tool (it comes for 15 days as free). Hope that helps you backup the Mac Mail message.
Sep 25, 2018 - 12:04 AM
Yes, you can easily take back up of all mails without any problem.
Use this reference: https://support.apple.com/kb/ph22318?locale=en_US
It is very easy and simple method provided by Apple.