We will utilize IMAP to transfer the local mails to Gmail which will make it possible in the end to simply move or copy mails to the Gmail account in the local email client.

The first thing that needs to be configured is IMAP support in Gmail. This is done in Settings > Forwarding and Pop / Imap. You only need to check the IMAP enabled checkbox to enable IMAP access for that GMail account.

The rest has to be configured in the local email client. Thunderbird will be used as an example but the basic settings are similar in all other email clients such as Microsoft Outlook.

Here are the values that you need to create the IMAP account in your local email client:

Email Address: username@gmail.com
incoming server: imap.gmail.com
incoming server port: 993
secure connection: SSL
incoming user name: username@gmail.com

outgoing server smtp.gmail.com
outgoing user name: username@gmail.com
outgoing server port: 587
secure connection: TLS

Google provides detailed (but sometimes slightly outdated) information about setting up IMAP in various email clients.

If the setup was successful you should check for new mail. All Gmail mail header should be downloaded to the local mail client.

To backup mails you simply move the mail folders into the Gmail IMAP folders. They will be created there with a label taken directly from the folder’s name. All folders and emails will be accessible in Gmail in the Labels menu.

It is even possible to take it a step further and delete old mails in the email client afterwards. If you need to access them again they are still accessible in Gmail. It is still advised to create a local email backup before deleting any mails locally.

If you encounter difficulties moving or copying folders you could circumvent the problem by creating the folders manually. The emails can then be copied or moved into the newly created folder easily.