Mail Problems - SMTP server not responding: "offline"

Question

I am currently testing DSL service for a month; set up Mail just before I downloaded the 10.7.3 release. Tested it once and it worked; did not use it to send emails since because I am only testing this (including the email address). I have been receiving emails (which I forwarded to myself from my cable provider's website); today, I wanted to send an email and a 6 hour troubleshooting saga began....
Total of 2 hours with Centurylink tech support; 1 1/2 hours with Applecare, and another 30 minutes with Centurylink.
During that time, all different settings were tried, turned on/off. POP server ok, SMTP server not responding. With Applecare, tried it on both the iMac and the Macbook; also tried it after booting into Snow Leopard - SMTP server not responding ("offline"). Applecare tech stated that it has to be the server since we tried it on two machines as well as SL. Centurylink insists there is nothing wrong with their server, but it's Mail's fault.
So, I finally gave up for the evening, but wonder if there is something amiss with Mail in 10.7.3 (both machines are on the same version) and during the troubleshooting, I noticed an anomaly: in a new message window, there was no choice to choose an account to send from which I thought was odd.
I was thinking of a new install or to clone back? My clones are up to date though, I have no older clones. I do have a fresh install but it's also the latest one from MAS. Or I could just boot from the fresh install and set up Mail to see if it would work?
Grateful for any ideas....

Answer 1

Thanks to everyone; as you can imagine, all settings/configurations were tried yesterday (even booting into SL); Apple said it's Centurylink's fault, and they insisted there was nothing wrong with their server.
FWIW, I unplugged my Comcast cable (it is still active though and I'm getting TV reception, just unplugged the modem for now); right now, my internet access and email account are with Centurylink for the test. I still access my Comcast mail via their web interface email account.
Anyway, thanks to a good friend's idea, I decided to check keychain access and to make it short: the issue is fixed and the problem definitely had something to do with the OS because:
When I first checked keychain, this is what I saw:
I then checked Comcast settings and there were two: one was checked "allow all apps..." and the other had the "confirm" with Mail listed under the applications.
So I figured it's so screwed up, I probably can't make it any worse, so I deleted the Internet Accounts and added Mail to the smtp.centurylink...:
And......... it then worked! These default "internetaccounts" obviously aren't being recognized by Centurylink's server?
And, to make sure that was it, I not only did the same thing on the Macbook, but also booted into a pristine "backup" fresh install (no added apps or files, just default OS), launched Mail and couldn't send. Launched Keychain, changed the apps to Mail, and I was able to send email.
I would call that a bug? Or not? In any case, I'll be calling back Applecare and tell them to add the above to the very long log from yesterday and to make a note that "internetaccounts" doesn't work with everything.