If you allow e-mail from any sender, you are bypassing the security settings for the {0}. This means that anyone could send ...

If you allow e-mail from any sender, you are bypassing the security settings for the {0}. 
This means that anyone could send an e-mail to the {0}'s address and their item would be added. 
With this option turned on, you are opening your {0} to spam or other unwanted e-mail messages.