After scanning, why am I receiving large amounts of emails containing an Acunetix test-string?

This problem of mass mailing is a vulnerability in itself. A hacker can perform the same steps to flood the mail system, for example by using automated bots. This issue is dependent on how the custom website actually works at the server side with certain type of requests. This mass mailing can be caused my more than one thing: Forms, Links, and multiple requests.

Acunetix WVS cannot predict if a website contains such entry points since emails are actually sent at the server side.

It is important to be aware that this can be exploited like a vulnerability to cripple a server. Such mass mailing entry points should be made more secure. When using forms for sending emails (example: registration forms), techniques such as CAPTCHA (http://en.wikipedia.org/wiki/Captcha) should be used to validate human input.