Do you have an auto reponse set up ? Please note an auto response might be set up in your webmail account accessed via a browser, so please check that as well. In Thunderbird Check out any auto sending of Return Receipts. Menu app icon > Options > Advanced > General Click on 'Return Receipts' When I receive a request for a return receipt: Is this set up to allow return receipts automatically or to 'Ask'? Do you have a 'Message Filter' which may be auto replying when emails are received ? Perhaps you set up an out of office type of response and forgot to stop it. This could be in Thunderbird, but it could also be set up in webmail.