What the message is telling you is that the Verizon mail server is not responding quickly enough. Your computer has contacted the Verizon server asking to send an email, and there's no response within the specified time period so your computer has given up waiting. As such it's not a problem with Thunderbird. It might be a problem with your internet connection to Verizon, or it might be a problem with the Verizon server itself. I suggest you try: a) connecting to a few web sites to check that your internet connection is OK; b) looking at the Verizon help page (if they have one) to see if there is any notice there about server problems.