Why won’t my email server allow emails to be sent to sbcglobal addresses?

At my work, I often email customers, and occasionally I get email addresses from sbcglobal.net

Anytime I email addresses from sbc, the emails never go through. I am usually just sending text, so it’s not an issue of attached files or large size emails.

I have also tested it with my wife, who has sbcglobal, and it is definitely not going through, even to a bulk/spam folder.

I can receive emails from these addresses, but I cannot send directly or even reply to emails I receive from them.

I don’t seem to have this issue with any other email servers.

Any ideas on a solution?