Most of my messages are being returned "unfortunately messages from (my IP) weren't sent. Please contact your service provider since part of their network is on our block list."
I spent an hour chatting with an AT&T agent; he clearly states that "RoundCube is a free email client support by all emails" but does not support mine "because it is the office email:" What does that mean? He did say that RoundCube needs to set up the IMAP setting??? Again what does that mean?
It sounds like your IP address is on a blacklist, are you running Roundcube on a remote server or on a local server?
I'm not sure what the difference is. I'm working from my computer at home as well as my computer at work. Both are running Windows 8.1
If you don't run your own email server, you'll need to contact your email provider.
If your Roundcube running on a server that is behind a dynamic IP address most mail servers will reject mail coming from it. If Roundcube is running a a remote server in a datacenter then most likely that server IP address has been blacklisted by email filtering systems.