As of June 20th 2017 we now offer IMAP, so migrating to the cloud will be much easier. We hate to see you leave but we want to make it painless for you. Perhaps you'd consider buying Office 365 or Google Apps from us?

Thunderbird Setup:

Install these plugin's:

https://addons.mozilla.org/en-US/thunderbird/addon/copy-folder/

https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/?src=search

Create user account in Thunderbird

http://screencast.com/t/dNoWMp6g4Z - mail configuration

http://screencast.com/t/ui23CIioi - check I understand the click Done

The following configuration parameters apply to your email client configuration:

Sync all non-folder emails first

Once all the emails in the root have been sync'ed then follow the next step to sync the folders

IMPORTANT: Before using Copy Folder, go to "Tools > Account Settings > Synchronization & Storage" and make sure "Keep messages for this account on the computer" is checked. If you did not previously keep your messages on the computer, you should wait until Thunderbird downloads your messages before copying any folders, otherwise you may experience duplicate copied messages.

Update MX records to use Office 365 or G Suite.

You can then copy the mail to Office 365 or G Suite folder as per the example below:

http://www.inmotionhosting.com/support/edu/everything-email/301-using-thunderbird-to-transfer-emails-between-servers