In order to send and receive email using Mozilla Thunderbird, you need to setup your account as described below:
  1. Start Thunderbird, go to the File menu and click New -> Account
  2. In the Account Wizard dialog box, select Email account and then click Next
  3. Enter your name as you would like it to appear for any mail that you send, and your complete email address (your-username@corporate-domain.com)
  4. Select the type of incoming server you are using as either POP / IMAP. 
  5. Login to your Webmail account. To access your Webmail interface, you can use the URL of the form http://webmail.corporate-domain.com. Here, you would need to login with your email address and the corresponding password. Go to Settings >> Forwarding and POP / IMAP Settings to obtain the corresponding incoming server name. Enter the Incoming Mail Server name and the Outgoing Server name in the corresponding boxes and click Next.
  6. Enter your full email address as the Incoming User Name and your Outgoing User Name and click Next
  7. In the Account Name box, you can enter any name that would help you identify this mailbox and click Next.
  8. Verify your account information and click Finish.

IMPORTANT
Usually, the port used for the Outgoing Mail Server/SMTP Service is 25. However, there might be a situation where your ISP (such as TM Streamyx and Unifi) might be blocking the use of port 25 for SMTP service. Many ISPs block port 25 in order to minimize the amount of spam sent using their network. If you are facing issues sending mails using the default port 25, you can confirm with your ISP whether port 25 is indeed blocked by them.

To circumvent this, you can use an alternate port 587 for sending mails if port 25 is blocked. Here's how:
  • In Mozilla Thunderbird, on the Tools menu, click Account Settings
  • Click on Outgoing Server (SMTP). In the Outgoing Server (SMTP) Settings section as seen in the below image, select entry for your domain name and click Edit
  • Replace the default port number 25 with the alternate port number 587
  • Click OK in the SMTP Server and Account Settings windows to save the changes