Set up your MailService account in Thunderbird
Click on the menu icon in the top-right corner.
Choose 'Options' from the drop-down menu.
Choose 'Account Settings'.
Click on 'Manage Identities'.
To add an alias identity to the email account you already use, just click on 'Add'.
Enter the name you would like to display when sending from your MailService address and add your MailService address as well. After clicking 'OK' you are done with setup.
Your alias is set, now you can send emails from your MailService address by choosing it from the drop-down menu where the sending address is shown when composing an email.