No, I don’t think you have to upgrade so much as you have to open a ticket with Mailgun and have them improve the quality of the free IP one is on by default, as I briefly mentioned here.
Perhaps that should be made clearer on the Ghost end since Mailgun certainly won’t be saying it out in the open.
That being said, wanting to get ahead of the curve with the second blog/publication I was setting up I pre-emptively opened a ticket with Mailgun so the email issue wouldn’t occur again. They wouldn’t reply until things demonstrably failed, I then opening a second ticket in order to make the request again.