I clicked the link you provided and navigated to the error code and was advised. <blockquote> The IP address you’re trying to connect from has an issue with the Domain Name System. Spectrum requires a full circle DNS for emails to be allowed through. Verify the IP you’re connecting from, and check the IP address to ensure a reverse DNS entry exists for the IP. If the IP address is a Spectrum-provided email address, contact us. </blockquote> I then asked google about the IP adress in the error The AI advises this <blockquote> The IP address 68.235.43.158 is registered to Amazon Technologies Inc. and is part of a network that serves cloud computing and other online services. The IP address was assigned by Amazon and is part of their AS14458 autonomous system. This information is typically used by network administrators to identify the owner of an IP address. </blockquote> My response is .... Turn your VPN off (it might be a part of your antivirus/Security suite). Or connect to the internet using the Spectum service as they refuse to send mail using SMTP unless you do. By the way you might note. None of this has anything to do with Thunderbird or it's capabilities. You might remember that next you are talking to Spectrum support and raise the terrible level of misinformation and just plain incorrect information they provide while claiming to offer support. Their server refuses your connection because the IP address you are connecting from is not issued by Spectrum. It is clear and reasonably simple but Specrum prefers to send you somewhere else to be informed of their policies. I would hazard a guess your previous inability to receive messages was also an antivirus product issue. They are the primary failure point in most folks internet connections.