That message is alerting you that your forum software has sent more than 25 emails in an hour; basically your host is only allowing you to send out 25 emails in an hour; which is very little...
In a nutshell, this isn't a threads bug, but a limitation imposed by the email server.
Example:
Every email validation email = 1 email.
Every email for a birthday reminder = 1 email.
Every email for a subscribed thread update = 1 email.
Every email for a pm notification = 1 email.
Now note that any user could get any number of those and you have a lot of email messages sent per hour...
