Importing from one gmail account to another

In theory, transferring existing emails from one Gmail account to another should just be a simple matter of turning on POP access to the source account, and then setting up the destination account to access the source account via POP. However, if this does not work, and none of Google's troubleshooting options helps, there is an alternative method that worked for me.

The error I received when I tried to attach the POP account was:

Server denied POP3 access for the given username and password. 

Server returned error: "[AUTH] Web login required: https://support.google.com/mail/bin/answer.py?answer=78754"

I've used the multi-platform Mozilla Thunderbird client previously to download all of my Google emails to my local computer for archiving. There is only one small additional step required to then copy these emails over to a new Gmail account. Here's the whole process:

• in the Gmail settings of your source account turn on POP access
• in the Gmail settings of your destination account turn on IMAP 
• download and install Mozilla Thunderbird (for Windows, Mac OS X and Linux)
• add your source account POP details into Thunderbird - it's even easier than the linked article indicates, just follow the wizard
• download your source emails into Thunderbird (click the Get Mail button)
• wait minutes or hours for all your emails to arrive
• add your destination account IMAP details into Thunderbird
• select all of your emails to transfer by clicking one email, then pressing control-a
• right-click the selection, choose Copy To, then pick your destination account and choose the folder of your choice, such as All Mail, or a previously created new label such as 'Old Emails'
• wait some minutes or hours for the emails to be uploaded to Google