Hello,
What domain is your niece sending emails From: and how is she sending them?
For example
something@hotmail.com w/ Mozilla thunderbird and uses xyz ISP
This sounds like she could be doing something her ISP doesn't expect and is sending using a server not covered by their SPF rules but I can't say for sure with that info and the full email headers.
Here is how to get the full email headers to help you troubleshoot the problem :
Full Email Headers

and welcome to Email Questions. -Raymond