Hi, As Firefox doesn't 'do' email, would you please take a look at this article : https://support.mozilla.org/en-US/kb/problems-with-email-how-to-find-help In case you are using Thunderbird, you can post your question on their support forum : https://support.mozilla.org/en-US/products/thunderbird (Click on 'Ask a Question' at the top of the page) However, if you're using Yahoo Mail - I found [https://forums.yahoo.net/t5/Delivery-errors-5xx-and-421/554-5-7-1-smtp-Error/td-p/413182 this thread] :