Thanks for the reply.
Using the Webmail interface just now, I tried sending a test message to myself. The response on hitting Send: "An error occurred! Failed to send message." That's all. I enabled all cookies for this test. I am able to view messages, though.
Using Thunderbird, when I hit Send for a message, I get a delay on "Connected to xepher.net...", followed by (in this case) "An error occurred while sending mail. The mail server responded: Service temporarily unavailable.. Please check the message recipient tsa-talk@lists.integral.org and try again." Same when sending to myself.