Hi Leaf, nice to meet the guy behind itch.io! Thanks for getting back to me.
Just tried again from two business domains and a Gmail account, and only the Gmail worked.
I'm gonna go ahead and assume this is an issue with my hosting provider. I use the same one for both of those domains, and my cpanel indicates an issue that's beyond my control. Going to have to get in touch with them.

That said, I'm able to email other recipients fine... just not to support@itch.io apparently! I've emailed the issue again with my personal account, and it has gone through successfully. The sale should indeed be appearing in those sections by this point, right?