SBCGlobal.net email randomly blocking my mail

I receive grading report emails from my daughter's online math curriculum, but my email account through AT&T/Yahoo+/SBCGlobal is randomly blocking some of them. I've checked my Junkmail folder, and they're NOT in there. I've checked my Blocked domains settings, and the sender is NOT on that list either. I've communicated with the sender, and they provided proof (below) that my email provider is randomly blocking their emails. "We've checked our records and we can see that the reports are being sent out successfully, but they are randomly being blocked by the end server: {graphic showing three emails sent to me, one "Not Delivered"} "Unfortunately, the errors are non-specific and just say "blocked" "You should be able to resolve this issue by reaching out to your email admins, but we have also found that this issue does not typically occur with gmail accounts if you already have one or would like to set up a gmail account specifically to receive reports for your students. " Please fix this problem! I want to receive the emails that are sent to me!