I use both Thunderbird and Outlook for my cox.net email account. I can receive incoming emails using Thunderbird but cannot send emails, and I can both receive and send emails using Outlook. The server settings are identical for both Thunderbird and Outlook - imap with STARTTLS and port 993 and smtp with STARTTLS and port 587. When Thunderbird is unable to send an email, I get a balloon that indicates that the smtp server cannot recognize my email address, which is [email protected].