Well, it's happening again, and I can't see that any solutions have been posted in the seven months since I asked last time, so....
I had been able to send mail on my Lunarpages account. Then suddenly, in the middle of things, having changed nothing, reset nothing, installed nothing, in the middle of a session this morning, I can no longer send mail through Lunarpages. (I could earlier, but when I went to send the next message, having done nothing in the interim but write the next message, suddenly I couldn't send any more.) Instead, I get the error message "550 in your email client", which has something to do with authorization. I haven't messed with any of the settings, and authorization is set up the way Lunarpages wants it.
For now, I'm having to use my local ISP to send my Lunarpages mail. I expect in a few hours or days everything will magically be okay again. But does anybody know what causes this outage? Is there a way of avoiding it, or should I just still with working around it?