I'm on the new BT Email software. I keep getting the error message "An error occurred when sending your mail. Please try again later" when trying to reply to a received message. You can click "OK" and try to send it again until you're blue in the face, the error just keeps popping back up. If I copy the text, close the message, select it again and paste the reply text back in, it now sends it OK. I get a similar "An unexpected error occurred" when trying to edit and send a saved draft message. I also find the new software is really slow despite having a 42M internet speed.
You may be being timed out by the email system.
Not ideal but when it happens try "refreshing" the page using the computer keys Fn F5. This will place your composed email into the draft folder where you can retrieve it and continue to compose or if finished to send the email.
It wasn't intended to fix the problem, it was a work around until the problem is fixed hence why I said "Not ideal.."