Summary:  Thunderbird is a widely used application for managing mailbox content. Users frequently encounter Thunderbird SMTP error, cannot send message which can restrict their functionality. Below are some strategies to help you fix this error. If this error persists, you should consider migrating to another mailbox such as Outlook. In order to easily migrate your emails from Thunderbird, you should access the PST tool with the help of professional software like Webbyacad M