At the end of the "new account" setup, Thunderbird may display a warning like this:

What does it mean?

This warning is displayed if you set up a mail server which does not use encryption (such as SSL or TLS) in order to protect your connection. The consequence is that (in theory) all your communication with this server (including login, password transmission and mail transfer) can be intercepted and read by a third party interloper.

What should I do?

First, you should check if your mail server supports encryption or not. Read its support and help articles in order to determine its connection settings.

• If it supports encryption, then you should change the settings in Thunderbird. Click the Change settings button and correct the settings.
• If it doesn't, then you should consider:
  • using it at your own risk (check I understand the risk and click the Create account button). Please don't hesitate to complain to your mail provider about this lack of security and request that it updates its service to a higher level of security.
  • OR switch to a mail provider which offers a secure connection (such as Gmail, Yahoo,...).

See also

TLS and SSL article on Wikipedia