Hi @Lacey Denton ,
I know it says we need to contact them, which we have done. Multiple people said they are accepting emails from other sources.
Being able to accept email from other sources doesn't necessarily mean it's not an issue on their end. After all, your users also have no issue sending mails to others.
With that being said, are you using Exchange Online in your organization? If so, run a message trace in Exchange Admin Center to get more details about the problematic mail. It might help determine whether it's an issue on your end or the recipients' end.
Message trace in the new Exchange admin center in Exchange Online
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