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Django only uses settings.DEFAULT_FROM_EMAIL when any of the mail sending functions pass None or empty string as the sender address. This can be verified in django/core/mail.py.
When there is an unhandled exception Django calls the mail_admins() function in django/core/mail.py which always uses settings.SERVER_EMAIL and is only sent to addresses listed in settings.ADMINS. This can also be verified in django/core/mail.py.
The only other place Django itself sends e-mails is if settings.SEND_BROKEN_LINK_EMAILS is True, then CommonMiddleware will send mail to all addresses listed in settings.MANAGERS and the e-mail sender is settings.SERVER_EMAIL.
Therefore, the only time a regular user will receive e-mail from your site is when you call send_mail(). So, always pass a real address as the from_mail argument and you will avoid users receiving email from settings.SERVER_EMAIL or settings.DEFAULT_FROM_EMAIL.
Side note: django-registration is at least one example of a Django pluggable that will send mail from settings.DEFAULT_FROM_EMAIL so in cases like this you need to make sure it is a proper e-mail address such as support@yoursite.com or webmaster@yoursite.com.