To begin with, I've already tried all of the tips listed under the following link: https://support.mozilla.org/en-US/kb/cannot-send-messages I.e., I deactivated the Linux Lubuntu firewall, I checked the settings of SMTP outgoing servers (which I didn't change; they worked before just fine and are correct), etc. Apart from that, the timeout error message appeared out of the nowhere since before everything worked just fine. So I didn't add a new email account to Thunderbird, everything had been set up and was working just fine, and from one day to the other the outgoing messages of any of my email-accounts wouldn't be set anymore. By contrast, receiving emails still works. I don't know what else to try anymore. I'm now sending emails from my phone or the websites, but it is very inconvenient not to be able to used the Thunderbird client directly for sending emails. Currently, I can only use it for receiving emails. Thank you very much in advance for your help on this issue. Kind regards, Andreas L. EDIT: In addition, I even installed another email-client called "Evolution" with whom I experienced similar problems. This made me suspect the network to be the problem. Hence, I connected my Laptop via WiFi-Hotspot to the mobile internet of my cellphone and magically all the emails of my numerous accounts are sent without a problem. --> How come, that a the local LAN-network somehow impedes me from sending my emails?