I am using Thunderbird 128.6 in an attempt to get away from Microsoft Outlook. I still use an outlook.com address though through IMAP on Thunderbird. Each time I send a message I am getting two copies in my sent box. I understand this is because Outlook sends an automatic receipt for the email and this gets filed in my sent box along with the original message sent. The receipt from Outlook is always larger in size than the original message, presumably because of the garbage added on to the header ID. An earlier post suggested filtering out these receipts if one can identify a standard item in the receipt which could be used to filter. I have established that the receipts headers always end with, in capitals, 'PROD.OUTLOOK.COM' but the filters in Thunderbird appear not to permit the header to be used for filtering. Any suggestions how I can use this identifying aspect to automatically delete these messages as soon as they hit my sent box?