There is no way to guarantee that you will be told that a recipient has opened or even received your message. Yes, there are services that claim to be able to do this, but they rely on hidden elements that "phone home" and even these can be thwarted by cautious or canny users. You can set Thunderbird to ask recipients to confirm that they have read your message, but this is an honour system and there is no compulsion on them to comply with the request. You can set a Return Receipt on a per-message basis. Look under Options in the menu on the Write window. You can set a default in the Account Settings, so this is per account. (I can't remember exactly where this is.) This settings page also has a button which takes you to a preferences page where you can set a global default. In the Write window options you'll also find a Delivery Service Notification which asks servers to report on processing of your message, but it is exceedingly uncommon to find servers that support this. It is viewed as a risk in that it could in theory be used by spammers to identify active email accounts.