My ISP is att.net. Was Comcast up to 3 years ago. As I recall sending worked with Comcast. The SMTP that works in Apple Mail is the one shown in the attachment. Mac Pro desktop, 10.10.5 24GB Norton Antivirus 7.0.1 Thunderbird 45 I actually have not been able to send email from T'bird for route some time. I usually just copy what I need, paste it in in Apple Mail to be sent from whichever account it needs to be sent from. I still use T'bird as an archive and backup. Here is the text of the error message: Sending of the message failed. The message could not be sent because connecting to Outgoing server (SMTP) smtp.mail.att.net failed. The server may be unavailable or is refusing SMTP connections. Please verify that your Outgoing server (SMTP) settings are correct and try again.