Have I been Hacked?

This morning my Yahoo email had a Microsoft one time code in response to a request I didn't initiate for a Microsoft account. I deleted and ignored.

Now I see on my Outlook phone app that I use to access my emails from my paid email domain, there are 2 Postmaster emails saying that 2 forwarded emails could not be delivered ot the recipient. I did not intiate any forwarding and the subject of the emails forwarded are legitimate from my 24 hr emailing history. The recpient was an email with a Tohru. org domain. The reason cited for failed forward is inbox full, and shows 4 attempts.

I'm freaking out now because I'm thinking someone hacked my email and forwarded all my mail to themselves and I'm only catching on because they ran out of space.

There's nothing in my sent folder to suggest these forwards, neither in my outlook nor if I log in directly to my mail domain provider on a browser.

If I've been hacked, is it my email domain that is compromised our something else? I've since changed the email domain password (which was already strong and created with Bitwarden). I always had 2FA as well.

What can I do at this point and is there a way to find out what if anything was actually successfully forwarded?