Weird error message

Occasionally when I try to post to Usenet I get the following error
message:

"437 Rejected 835 000001 IOErrorX"

and my post gets saved in "dead.article" in my home directory. If I keep
trying several times, all within the same few minutes, it continues to
fail. But if I try again a few hours later, it usually succeeds. Doesn't
matter which newsgroup I'm posting to, it's happened in several groups,
at all different times of day, etc. No pattern that I can discern.

Anyone know what causes this? Should I complain to my ISP?

Thanks,
Joyce