Thunderbird Outgoing Authenication

[seacue]

I know I'm stupid, but......

I use two ISPs.

One does not require email authentication and works with Outlook Express and Thunderbird.

The other requires authentication. It does not require nor accept secure password authentication.

It works fine with Outlook Express if I check the box which says my ISP requires authentication of outgoing mail.

I can read incoming mail with Thunderbird but the ISP claims that I'm not doing authenication when I try to use Thunderbird to send an email. I can't find an option like the OE one for authentication when sending.

Any suggestions?

I'd like to get rid of the dialup ISP that doesn't require authentication and thus works with Thunderbird.

thanks
seacue