A screenshot of the error message you get when sending would be more useful.

I'm sorry to say but there is no error message. Thunderbird puts the mail in the "Sent" Folder, just as if it would have been sent. Discovered that yesterday after a phonecall from my insurance compagny that was waiting for un urgent document.

Send a test message to another email account of yours. See if the message gets delivered.

First of all, thanks for your answers Stans ! Sending mail to my gmail account or to my girlfriends account is not deliverd, but still, as i said previous, Tbird puts the message in the Sent-folder as if it was sent... Just changed my pasword (as I read in an older answer of you on someone's question), but no solution either.

Ok. Go to Account Settings > Outgoing Server (SMTP) Settings and delete the smtp server that is in use by the affected account. Close the dialog, restart Thunderbird then go to the same section and add the SMTP server afresh. Make sure to use your service provider's settings. Next, open Tbird's password manager and delete the entry for your provider's smtp server, then try sending the message. If all goes well, Thunderbird will prompt you for the password to log in to the smtp server. MAke sure you check the box to make Tbird remember the password.

Stans, a million thanks to you and your knowledge, it worked !!!

Many, many, many thanks, you saved me from hudge trouble concerning the mails i need for my job.

Don't know how to thank you enough !!

Geert.

I'm glad it worked out. You're most welcome.