Open up Thunderbird then click on the tools menu at the top, then select 'Account Settings' from the drop-down menu.
Click on the 'Add Account' button.
Choose email account and click next.
Once the new window appears, input your name (This is used on all mail you send as the sender of the mail - e.g. From: John Smith) and your email address. Once done, click next.
This step configures what servers to use when you are sending mail and receiving it. The settings should be set to the following (Once done, click next):
Incoming mail server type is POP. Incoming mail server is mail.yourdomain.com (this may vary, please ask if unsure)
Here you enter your username, which we have given to you and click next.
Account Name describes the mailbox (This is your choice). Once you have given an appropriate name click next and then click finish to go on to the next step.
Now go to the 'Outgoing Server (SMTP)' tab. If there is already a record displayed for mail.yourdomain.com then click it and then press 'edit' otherwise click 'add' and then type mail.yourdomain.com (this may vary, please ask if unsure) as the server name.
Make sure that 'Use name and password' is ticked and again enter the username, which we have given you. Make sure that 'Use secure connection' is set to 'No'.