Need help with sending, getting "Too many messages"

I've been sending several books to the the one email address over yesterday and today and all went well. However, this afternoon, 10 went successfully but every time I've tried since has failed, whether 10 or just 1 to that address or my own.
I don't know if it's an ISP problem or something wrong within the book(s) I'm trying to send. I've kept a .txt file of the last 2 times ...one for 10 ... and one for one book. Can I send it somehow to get advice on the problem? And if so, to where?

Many thanks.