emails from hotmail addresses suddenly not reaching us

Ask one of the hotmail senders to forward you the bounce message they get back, if they get one. The error code in it tells you whether your mail server rejected the message or Microsoft never sent it, and that halves the problem. If they get no bounce at all, the mail is reaching your server and being dropped or quarantined there. Check your spam quarantine and any filtering service in front of your mail, a rule or blocklist update can catch Microsoft's sending servers by mistake. Did anything change on your side recently, new filter, moved host, DNS change?