• Open Thunderbird
• Click on Tools at the top
• Select Account Settings
  

• Select Outgoing Server (SMTP)
• Enter mail.domain.com
• Select Use Name and Password
• Enter the first part of your email address into the Username field
• Click the Add Account button

• Select Email Account
• Click Next

• Enter your name into the Your Name field
• Enter your email address into the Email Address field

• POP should already be selected
• enter mail.domain.com into the incoming mail server field
• Click Next

• Enter your entire email address into the Incoming User Name field
• Click Next

• Click Next

• Click Finish

Thats it!! You should now be fully configured to send and recieve email with your realtime.net email address.