In the Mozilla Thunderbird application, when trying to send an email from several of my email accounts, the following message appears: Error sending message The message could not be sent because the connection to the SMTP server smtp.gmail.com failed. The server may be unavailable or is refusing SMTP connections. Please verify that your SMTP server settings are correct and try again, or contact your network administrator. I called my hosting provider, and they told me that everything is fine on their end. I can't figure out where the problem is, and all my settings seem to be correct. I don't know how to resolve this.