The autoconfigure routine usually looks for IMAP settings, but it appears online.nl only offers POP access, so you should click the Manual config. button after you enter your address and password in the account setup window (File/New/Existing Mail Account).

It looks like the settings are pop.online.nl for the incoming POP server on port 110, connection security = none, authentication = password, transmitted insecurely, user name = email address. For the outgoing server, smtp.online.nl on port 25 (or 587?), connection security = none, authentication = password, transmitted insecurely (or none?), user name = email address.

Thanks, but doesn't solve my problem. Thunderbird-notification remains: can not find settings for this account.

You have to click the Manual config. button before TB tries to find the settings, which it can't in this case. Or, go offline to bypass the lookup routine, enter the settings manually, then go back online, File/Offline/Work Offline to switch between offline and online.

Solved! Thank you