Weird new email sending problem

Hello --

In the last few days, I have been having a very annoying problem with mail. I'm on a Macbook Pro using the generic email program.

I use my ISP's info for SMTP server info. It has always worked fine. Now, when I try to send mail, it tells me it can't send from that server. Connection doctor shows it's connecting fine. But it still won't send.

However, if I delete and recreate the SMTP server with the same name it already had, using all the defaults as before, it works fine for a while. Then the problem occurs a day or so later.

WTF?

Thanks -- Barbara