Can not send email to group

Hello,

I created a group and placed in it a number of my own personal emails to test. I am unable to send email to the group with one of my email addresses which is a "mac.com" address. I receive this from postmaster@mac.com:

Your message cannot be delivered to the following recipients:

Recipient address: Bulletinreview@dgsda.org

Reason: Remote SMTP server has rejected address

Diagnostic code: smtp;550-Please turn on SMTP Authentication in your mail client, or login to the IMAP/POP3 server before sending your message.

Any clue what this means and how to resolve it?

Thank you for your help.