I am assuming this "hidden copy" is a BCC copy of the email. It should be a simple matter to look into the sent mail and see if the BCC entry is there. Unless you are using Gmail that is.

Hi matt and thank you for your answer! the emails are being received by the receiver, but the BCC is not delivered to the onepage crm email. i even tried to send a test email directly to the capture email and thunderbird didnt send it. we are using loopiawebmail as a provider and when i try to send a BCC from loopiawebmail it went to onepage, so i guess the problem is between thunderbird and onepage crm.

''WI [[#answer-1292223|said]]'' <blockquote> so i guess the problem is between thunderbird and onepage crm. </blockquote> No, a simplistic view unfortunately. When you send mail, it goes to the SMTP server I assume for loopiawebmail as they are who you mention. While this is happening, thunderbird has up that sending mail dialof with a cancel button. Unless an error occurs with the server, which will replace the sending dialog, the quite removal of the sending button indicates the server accepted the mail for delivery. I asked you to check the send folder that the BCC was correctly inserted. So the mail that was offered to the SMTP server had a BCC field. As there were no errors loopiawebmail accepted the email, including the BCC for delivery. You will have to ask them what is happening there, but it would not surprise me to hear they were not sending BCC emails because the BCC field is often used (or used to be) for SPAM deliveries. So at this point, you have really three choices. #Contact loopiawebmail and ask them to check their SMTP logs for the results of your attempts to send mail through their SMTP server. # Log the SMTP connection to ensure the email is correctly send and accepted by the server. https://wiki.mozilla.org/MailNews:Logging # Try the various safe mode options to exclude third part applications. *Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender. *Restart the operating system in '''[http://en.wikipedia.org/wiki/Safe_mode safe mode with Networking]'''. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: [https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode Windows 10], [http://windows.microsoft.com/en-us/windows-8/windows-startup-settings-safe-mode Windows 8], [http://windows.microsoft.com/en-us/windows/start-computer-safe-mode#start-computer-safe-mode=windows-7 Windows 7], [http://windows.microsoft.com/en-us/windows/start-computer-safe-mode#start-computer-safe-mode=windows-vista Windows Vista], [http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx?mfr=true" Windows XP], [http://support.apple.com/kb/ht1564 OSX] ; If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates. If you need help reading the log, I will try and assist. But do not try and post it to this forum. It will be long. Whilst I listed safe mode last, I would suggest try it first, as anti virus tools these days think they are more than they are and try and control all sorts of aspects of email, including BCC usage.