I have this problem for posting, PMs, mass e-mails, everything...
-This didn't happen previously, and no settings have changed. The e-mail settings are correct.
-I host myself.
-The posts/e-mails usually work despite the error message.
-If a user posts and gets this message his own post will cause the topic to show up as new post that he hasn't read yet.
-This isn't always a problem, it comes and goes.
I don't think I want to just eliminate the error message because I don't know that that fixes the problem so much as masks the symptom. I would just ignore it, but many of the users don't realize their posts are working and post multiple times.
This has been happening off and on for a while. I work at it a little, then give up. Then try to look into again when it starts happening again....then give up again...