We have this same exact problem.
It's gotten worse as time went by. I thought it might have been our old NT 4.0 server, and yesterday we completed migration from old server to new server (forums were the last thing to move.) I was hoping this would solve the issue, but it has not.
We have numerous things that send mail from this server using Net::SMTP module that never have a problem, other mail functions on the board never have a problem (password request, moderator notification, etc.), it's just reply notification and it's sporadic.
Tim
