Hardly anyone can receive my messages

[MMcK]

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?

[SKaero]

It sounds like your IP address is on a blacklist, are you running Roundcube on a remote server or on a local server?

[MMcK]

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

[ABerglund]

If you don't run your own email server, you'll need to contact your email provider.

[SKaero]

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.