I can't send any e-mail to a @gmail.com account. The error message I receive says that my mail is unauthenticated and suggests that I "set up email authentication for your domain" but I do not know how to do so. Any help would be appreciated!
I'm also confused if this is a roundcube setting or a webmail setting!
Thanks so much everyone!
Roundcube - its just web-based mail client.
You must setup your SMTP server -google about DNS, SPF, DMARK, DKIM, SSL-certs.
What gmail error you see? Copy it here.