There are 3 different services that may be configured depending on what you want to do:

• Create a POP3 account: This will allow Thunderbird to go and get e-mail from the mail server and download them to your computer (classic e-mail use).
• Create an IMAP account: This allows Thunderbird to get e-mails from the mail server without deleting the messages from the server (webmail use)
• Create a SMTP account: This allows sending e-mail through Gandi's mail server

In the large majority of cases, one uses an incoming service and an outgoing service such as either POP3 & SMTP or IMAP & SMTP (the coupling of POP3 & IMAP would not be of much use for the same account :)

See the guides:

• Creating a POP3 account on Thunderbird
• Creating an IMAP account for Thunderbird
• Creating a SMTP account for Thunderbird