iMessage on Mac Receives, but Can’t Send

I have six iCloud users on Macs. Upgraded one Mac to Tahoe, and now, that user can receive iMessages from the other five users, but can't send a reply to three of them. The reply just says, “Unable to send.” Also, if a new message is created to either of those three users who can’t reply to, their Apple ID (email address) shows up as red. Yet again, can receive iMessages from them! This is only sending messages between Macs (in a department).

I'll assume this is an iCloud network issue?