Hello there We have read your mail. We try to help you. Yes, the problem arises as to whether the e-mails can normally be sent and every once in a while the error is reported again. this also gets worse because it is indeed possible. The code actually contains that the use of the server through which e-mail is sent suffers from spam and other virus-related topics. The service provider is protected from the outside for these. You can certainly check the antivirus well if possible, this also applies to firewalls etc. Please read this article http://kb.mozillazine.org/5.7.1_Unable_to_relay I will look further into thunderbird for you. Can you explain how long you have had this and what type of computer do you have? What is the provider with you? Is this a well-known brand name? And where for your using the mails for? Is it a single reciever of this mail s or large groups or internal group?. Can you contact receivers if possible? Especially under the moderator function. Get a good antivirus soon as possible. It s about a cause : security leak or error type.(as reported). no permission from the other side to receive data such as emails etc. Mail us back please. Greetings Firefox Volunteer