This is not a Thunderbird problem. A couple of possible reasons. You sent an email to a valid email address but it was not the correct email address eg: a typo So someone else received the email. Check the email address you used to send TO brother, the copy in Sent folder would show it. The email was received by BT but it got put into the server Junk/Spam folder. If your brother uses a POP mail acount then Pop can only see what is in the Inbox and download to pop mail account Inbox. So, it would not get downloaded. Brother would need to logon to the BT webmail account to check if anything is in the Junk/Spam folder. If it is in that folder then he would need to set it as Not junk and perhaps modify settings to always allow your email address. So emails go his Inbox and stay there. If your brothers webmail account has reached a full quota then it cannot receive any emails, but usually you would get a message back from the server saying it could not be delivered. Again, if you are using a Pop mail account and that mailor daemon email got put in your gmail junk/spam folder then you would not see it unless you logged on to the gmail webmail account.