Outlook Hostname Mismatch Troubleshooting

I'm experiencing a hostname mismatch error with Outlook on multiple devices. This issue is not related to any recent changes in my network or email server settings. we have issues around hostnames and outlook certificate errors. its been a mainly ongoing experience since our last server admin died. Our new server admin doesnt seem to be able to sort it so we are posting job to find an expert who can. we run our own in house mail server, (mailcow) with 2 different domains. everything has been running ok mostly until the last certificate expired and since then, we get pop ups on outlook everytime we open it. this is preventing some users from running outlook on mobile devices etc. Ideal Skills and Experience: - Expertise in email client troubleshooting, specifically Outlook - Network configuration knowledge - Experience with SSL/TLS certificate issues - Strong problem-solving skills - Excellent communication skills to explain technical issues in simple terms