Your message was sent but a copy was not placed in your sent folder (Sent Mail) due to network or file access errors. You can retry or save the message locally to Local Folders/Sent Mail-xxxxxxx@gmail.com

As the message says, your mail was sent. You can always ask the recipient though. I recommend disabling the option to save a copy of sent messages in Thunderbird, for Gmail, because Gmail's outgoing server automatically saves a copy of sent messages in the sent folder and you don't need to activate the same action in Thunderbird. It will also get rid of that error in future messages that you send.