POP Mail server connection errors from outlook

I just set up (4) clients with zoho mail over the last several weeks. All accounts had been working with no issues.

Today all of my clients are complaining of Outlook POP server connection issues with ZOHO mail.

We have not changed any settings. I do not see any posts by ZOHO with regards to POP mail issues.

Clients can log into ZOHO mail remotely...but cannot gain access via outlook.

We need zoho to look into this ASAP....as this will severely limit out Clements on Monday AM....work week.