Hi Christina,
I understand when iPad is connected to the Internet you can send emails without issues. It indicates your Office 365 email service works well. I have tested the issue and reproduced it. Based on my experience, this issue could occur if when you reconnect the Internet and the iPad don't send the email in the Outbox automatically. And this could be a behavior of iPad side. As a workaround, I suggest you send emails when connected to the Internet or manually send the emails in the Outbox after reconnectting to the Internet.
Your understanding is appreciated.
Thanks,
Xinyu