How do spam email accounts get onto devices?
Check http:// minbox.email/join#/ (not linking them here though, don't want to generate traffic to their site); they offer a free email inbox in exchange of receiving spam. As of how they configure the email account on your device, it appears that it is made through a Configuration Profile.
I tried to verify myself by creating an email but couldn't end the process because of an error on their end.

You or anyone can create a configuration profile to


  (...)quickly distribute groups of settings and access powerful
  management features that aren’t normally available.


Then 


  You could email the configuration profile to someone–including
  yourself–or offer it for download on a website. When someone taps the
  email attachment in the Mail app or downloads the configuration
  profile file from a website, they’ll be prompted to install it on
  their iPhone or iPad.


(from this article).

Note that your mother would have had to accept the prompt in order to apply the Configuration Profile.

It's also important to understand that this may be a vehicle for malicious configurations as explained in this article.