In Thunderbird, click the Burger menu icon in the upper left corner and then click Account Settings.
In the newly opened tab, click Accounts Actions in the bottom left corner, and then click Add Mail Account....
Under Your Name, enter the name that you want the recipient of your emails to see as the sender. This can be your name or any other content.
Enter the Email address which you have created before and for which you want to set up the mailbox (e.g. firstname.lastname@example.org).
Enter the Password that you assigned when you created this e-mail address and click on Configure manually.
Select whether you want to retrieve the emails using IMAP or POP3 protocol:
POP3: emails are retrieved directly to your PC's hard drive and removed from our server.
IMAP: You will see the same emails on each device used to retrieve the emails. If an email is deleted, it will also be deleted on another device. This requires more storage space as the emails are stored with us.
Incoming and outgoing mailserver: This is displayed under the Hosting menu item (left side menu) in your customer area. Enter only the server that is displayed under Hosting.
You can leave this setting on Auto.
You can set this setting to Autodetect.
You can leave this setting to Autodetect.
The username is the full email address for which you are setting up the mailbox.
Finally, click Re-test and then Done.
Your mailbox is now set up in Thunderbird and ready to use!