That actually looks like an anti virus might be blocking outgoing mail. does your anti virus have any mail controls?

Thank you for your answer, Matt. I am not sure if there's email control in my antivirus,but I tried turn it of without success. I also turned off Windows firewall and bit defender firewall (it comes with my avast antivirus), again without success. I also uninstalled and reinstalled thunderbird and removed/added accounts again. Nothing worked.

OK, I contacted my ISP provider today and they told me that my outgoing emails were blocked, because my ip has been sending spams! I did definitely not spam anyone, so it's obviously a virus that has gotten into my PC! Hopefully my old PC, I am on a new one now. I am just worried because I had all the security measures on (antivirus - firewall) and I yet got a virus! I know this is not the right place to ask, but what measures do you recommend to avoid that in the future?

Do not be sure of the issue being a virus or malware. With cheap hosting providers you share an IP address with others, so you get caught up in their actions. (it is called a bad neighborhood when they are bad apples).

The only way to be sure you are no caught in such an issue is to actually have dedicated server hosting. This is significantly more expensive that the $12 for 12 months deals, starting around the $50 per month.