Is there potentially a firewall setting in Mail that is preventing my message from sending? ... All the Mac help articles point to having the proper settings which I triple checked with my email provider....
If your outgoing mail (SMTP) port is 25 then that may be what is blocking your original messages. Nowadays, many ISPs block port 25 to reduce spam and bulk sender messages. Apple's KB article lists alternative ports 465 and 587, however you should check with the ISP that hosts cabec.org for their recommended port.
Note: You can use the smtp.att.yahoo.com info for sending.
I have no clue as to why it works on your iPhone and not on your iMac as I have a similar issue with one of my email accounts that use the same host for five other accounts.
Thanks for your suggestions. After trying various methods, including contacting the host server tech support and verifying all the settings, I took up the issue with AppleCare tech support and they're on the case. They weren't able to easily solve it and continue to work on it. The first time I've really used them for anything and I was shocked, in this day and age, to actually have someone on the line for over 60 minutes working through a problem - rather than going the route of most companies that put you through an automated voice system only to wind up talking to an impatient high school dropout. Totally impressed by the AppleCare support!
thanks again for your suggestions.
I unfortunately have the same problem of not being able to send email too. I called AppleCare and the tech support was not helpful at all. I was really disappointed with the service. What a waste of 30 mins and my money! Hoping to call again another day and perhaps I will get a more helpful person on the line.