The prompt is when the email opens with invoice attached. when you click on 'send' it is sent as is.

Have you noticed if it is always selecting the 'default' mail account? If yes, then I suggest you try changing your current selected default mail account. Right click on mail account in FolderPane and select 'Settings'. The mail account at the top is usually the one currently set as default. To choose another - select the mail account name click on 'Account Actions' select 'set as default' click on 'OK'

Another idea would be to put Thunderbird in 'Offline' mode before doing invoices, then the 'Sent' emails will be put in the 'Outbox' as a 'Send Later'. This allows you to check the emails before going back to 'online' mode and sending.

There are a couple of add-ons that have options to check the sending account:

https://addons.mozilla.org/en-us/thunderbird/addon/identity-chooser/

https://addons.mozilla.org/en-us/thunderbird/addon/correct-identity/

Thanks guys. After reading your comments i was able to come up with a solution. I installed "identity chooser" which helped by changing the colour of the sender, alerting me to look first, however i concluded that i might still, in my haste press send accidentally.

So, I added another email account to the list and made this one the default account. so now every time my invoicing program opened up Thunderbird that account would be default.

BUT........ I hear you say, this would still have the same effect. BUT NO!! my friends. i decided to alter the security settings and authentication so it keeps looping trying to send and failing. Thus alerting me that i am still a cock and have forgotten to choose the correct sender email.

TADA !! for now this looks like the perfect fix.

Thanks for the help.