I opened a ticket with Microsoft support because the invitation emails are going to the Junk folder for Outlook.com users.
After some back and forth with Microsoft they decided they are still unable to provide any mitigation for this issue, but they did suggest the following tips to help resolve the problem:
Tips for getting email delivered to the inbox at Outlook.com | Email Questions
In the meantime, if you request an invitation to an Outlook.com address, please be aware it may deliver to the junk folder.