Sorry - We sometimes have issues with some e-mail name servers - Most notoriously Yahoo and Mail.com
The "symptom" is a bounce message from MAILER-DAEMON that states "No MX or A records for shubertnm.net" or "No MX records for shubertnm.net"
The cause is Yahoo and/or mail.com (gmx.com) have issues keeping their name servers up-to-date. Yahoo, on occasion, has "forgotten" hotmail.com and microsoft.com to name a few.