Oh wonder......After a few hours things are working like usual! Ziggo denies any problems with their smtp server but I tend to question that ; -).

Assuming your settings are correct (smtp.ziggo.nl on port 587, STARTTLS security, authentication = normal password, User name = full email address), the best way to see if Avast, or any other program, is interfering with email is to run in Windows safe mode with networking, and then test TB performance. In general, it's recommended to disable any AV scanning of email (e.g.add the TB profile folder as an exception) and any AV monitoring of secure email connections.

Thanks for the reply! I found out what the problem is. I use a VPN connection and THAT is the problem. When disconnecting from de VPN everything works perfectly. I tried to exclude Thunderbird from using the VPN conncection but this didn't work either. So disconnecting from the VPN when sending mail via Thunderbird is the only option I'm afraid.