If I was you, I would be looking into possible BAdI, user-exit or enhancement solution. I am not an expert on this area, but I would assume that there are different possibilities to have your own code e.g. during a sales order is created and credit limit exceeds. Doesn't it even issue an warning/error message when this happens? If this is too early situation to send the email, you might want to start a workflow and set it up to wait until next day (or whatever), and then check in the workflow whether the situation is still valid, and then let the workflow to send the email.
A background job solution might work as well (or actually be a better solution depending on your requirements). Just need to take care that you will not send the emails repeatedly to the same customer, etc.
Regards,
Karri