Microsoft Teams - Employee receiving meeting invites without being invited to meeting

Hello,

Our company has an employee using an Office 365 account outside of the organization that does not include Microsoft Teams, so he is using a free account to communicate with us. We are unable to add him as a guest, so he is only able to communicate with us when we schedule a meeting and send an invite to his company email address.

For some reason, whenever we schedule a meeting in any of our channels, he receives the invite without being invited. We have explicitly confirmed that we did not type his email address into the invite list when we are creating the meeting, yet he still receives an invite anyways. I'm not sure if it's something in his settings, or it is something within our organization. Can someone shed some light on this situation please?

Thanks!