Have your correspondents checked their spam folders? Your ISP or your IP address range may have been blacklisted. With many email providers, sending to yourself will be permitted and will confirm (or not!) that between them, Thunderbird and your SMTP server are doing their job. Some email providers, such as gmail, think that you don't need to receive a copy since you already have one or more in Sent and All Mail. In this case, it's useful to have a second email address of your own to send to.