I co-locate a server with my local ISP (PacInfo) and have my email client set up to send using the SMTP server of my base domain. However, my DSL connection is with Qwest.
It never occurred to me that Qwest might be the culprit.
So the first thing I did was close Thunderbird, restart it and try again. No go.
Next, I reboot my computer. Now, I turn my computer and peripherals off every night, so that shouldn't have been it. And it wasn't... Still no outgoing email.
That left me with my web server. Well, it's the middle of the night, so I wait until today to reboot my server. That way, if anything screws up I can always call on Randy, the ISP's owner and tech extraordinaire, to bail me out without feeling guilty.
Still no outgoing email.
So I break down and call Randy. He solved my problem in under a minute...
Apparently, Qwest is no longer allowing email to use port 25 if your email server is not Qwest's SMTP server! I think it's great that Qwest is doing their part in combatting spam, but a heads up would have been appreciated!
I don't know if this is a unilateral change, or if they are rolling it out in stages. But if Qwest is your ISP and you don't use their outgoing mail server, you may experience the same issue I did.
The cure is to contact your webhost for the domain you are using for outgoing email, and find out what port they use. Plug that value into your email client and your outgoing email should start working again.
I hope this saves you from the low level panic, and high level frustration I went through.