my DNS has no entry for that server name. That s exactly the issue that Thunderbird is complaining about. I checked against the IP address and that would indicate that is is in the vicinity of Oslo, but the domain is no longer registered.

So you could change the domain name to the IP address, or work out what happened to the DNS entries for that domain. Perhaps consult the hosting provider.

Thanks Matt,

before posting the question I had of course done some research and contacted my email provider. Whois did not resolve the pp.htv.fi but did for the main domain htv.fi. My email provider rep suggested after some tests that the culprit is the Thunderbird client. I had used my account over 20 years with other clients and Thunderbird also a long time and the problem dropped just out of the blue. I could not change the domain in the email address because my correspondents could not response to me if I did. Well, I did drop the pp from the domain and put it back again and lo & behold the thing started working! Now, I do not know if T-client checks the senders domain before sending, or my clients settings have changed or my email provider has done something, but it works now. I am flabbergasted but happy! By the way domaintooks.com resolves the pp.htv.fi into htv.fi now, I did not check domaintools.com before. Go figure...