POP or IMAP? I have never felt any motivation to use Send Later. I don't have a use case that requires delayed sending. It isn't helping here because you don't seem to know exactly when any particular message was sent. If you use the ordinary "send immediately" mode then if a message fails to go you are left in the Compose window with your message shown, and you're free to retry or save it as a Draft. Send Later seems to interfere with all that. I'm wondering if the message about spam is immediate (from your SMTP server) and so is treated as a send failure. In that case your message not landing in the Sent folder is sort of explicable. OTOH, if the spam detection is occurring some time later, or even coming back from the recipient's server, then at the point of sending your message would have been successfully handed over to the SMTP server and so ought to be in your designated Sent folder. And that's why I asked about POP vs IMAP. If the account uses POP or the Sent folder under Local Folders, then the management of it belongs to Thunderbird. If that Sent folder is on an IMAP server then arguably the server has an opportunity to deny the writing of the message to that folder. I find it incredible that when they reject a message they don't copy it back to you. Have you looked at the source of any of these spam refusal-to-send notifications to see if the original message is being quoted back to you? Maybe the message is improperly constructed, preventing Thunderbird being able to display it. Use ctrl+u to see a message's source.