Once the news came out that Thunderbird was no longer going to be actively developed, I knew it was time to switch. Whilst the program will be maintained it will only be a matter of time before it falls out of favor and in the Tech industry you cannot stand still (for long at least).
First thing to establish is where the email resides. I personally used POP3 to retrieve my emails which downloads the emails locally and archives the one on the server (in my case Gmail). This meant all my emails where available locally.
Another popular protocol is IMAP which accesses the emails on the server but certain amounts of emails can be cached offline for quicker access. Using this protocol means the emails will be stored on the server and there will be no need to upload the files. The only caveat is that the emails can be copied or moved from the server to the local computer.
The following will use the IMAP protocol to upload the local emails to Gmail. This works really well and keeps the folders to labels in Google intact. The only manual work is selecting the folders to upload.
Sign in to Gmail.
Click the gear icon in the upper right, then select Settings.
Click Forwarding and POP/IMAP.
Select Enable IMAP.
Configure your IMAP client and click Save Changes.
Go to the Gmail help page and select I want to enable IMAP . Follow the on-screen instructions to add Gmail IMAP to Thunderbird.
There will be at least 2 accounts in Thunderbird now. The local / POP account and Gmail. Each account have the same folders like Inbox, Trash, Spam, Draft, etc. As this method works for IMAP to IMAP E.g Gmail account to another Gmail account the accounts will be called “source” accounts.
Use the Ctrl key and click on the folders to highlight them for the source account E.g local / POP accounts.
Click and drag the folders to the Gmail account under the Inbox folder. This will begin the transfer. The status bar at the bottom will have an indicator of how many there are and how many are left to upload.
Repeat until all the folders have been copied to Gmail. Verify them in Gmail to double check. Also it is best not to do them all at once in case it disconnects or the application crashes. At that point some will be uploaded and others have not and it will be a case of checking what have and have not been copied.
The Thunderbird method is a cheap but manual method of copying emails. There no additional software required if Thunderbird was already installed.