Thunderbird does not always send the reply message
It has happened several times now that a message that I replied turned out not to have been received by the person I replied it to. This although Thunderbird indicates that it has been sent.
Todas as respostas (1)
That's quite possible, for whatever reason. Once the message has been sent from your Thunderbird it's beyond your control what happens, and certainly not a Thunderbird problem. It may be an issue at your email provider's server, at the recipients email provider server, or at the recipients computer/email client. The first thing I'd do is to ask the recipient to check the spam folder.