Here are the instructions on how to setup Mozilla Thunderbird with your FlexBox account. The attached PDF is a step by step guide to configuring your mailbox. The basic settings without instructions are as follows:
Username = user@example.com
Incoming Server = flexbox.mxtoolbox.com
Outgoing Server = smtp.mxtoolbox.com
Advanced Settings Incoming Server
Leave messages on Server (this will use storage quota, but will allow you to use the web based client as well as other clients)
Incoming Server uses TLS
Advanced Settings Outgoing Server
Username = user@example.com
Outgoing Server uses TLS
Peter
Product Development Engineer
peter@MXToolBox.com
512-637-8762