JohnDoh,
Thanks for the reply. I am using my own SMTP server. I have no trouble using my email server with the Mac mail client which was configured to use a TLS certificate. that's the only thing that I see different from roundcube's configuration. Can roundcube be setup to use TLS? Why else would email be bounced back when sending through roundcube and not another email client? I have contacted my DNS provider, No-IP and they have suggested I use their Alternate-Port SMTP service.
Thanks.
-Tim