Question : Bouncebacks from distribution group

A user recently received the following bouncebacks from a distribution group. This distribution group contains only 'external contacts' in Active Directory, with SMTP addresses. Sending to this distribution group reproduces this issue, but emailing individual contacts does not produce a bounceback. I have replaced occurrences of our domain with 'example' and redacted certain information from email addresses and names. Please let me know your thoughts on this issue, and if this is a possible spam filter behavior since there are many recipients.

Note: extra right bracket in Bounceback3 is exactly how I received the bounceback, but I have checked the contact and the extra right bracket is not in the email address.

_Bounceback1________________________________________________________
Your message did not reach some or all of the intended recipients.

      Subject:       Meeting reminder
      Sent:      5/10/2010 3:57 PM

The following recipient(s) cannot be reached:

      Marc [redact] on 5/10/2010 3:58 PM
            There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            <mail.example.org #5.5.0 smtp;550-mail.example.org [ourpublicIP] is currently not permitted to relay>

_Bounceback2________________________________________________________
Your message did not reach some or all of the intended recipients.

      Subject:       Meeting reminder
      Sent:      5/10/2010 3:57 PM

The following recipient(s) cannot be reached:

      Steve [redact] on 5/10/2010 3:58 PM
            Unable to deliver the message because the originator prohibited redirection to an alternate recipient or the message was encrypted
            <mail.example.org #5.7.3 smtp;530 5.7.3 Client was not authenticated>

_Bounceback3________________________________________________________
Your message did not reach some or all of the intended recipients.

      Subject:       Meeting reminder
      Sent:      5/10/2010 3:57 PM

The following recipient(s) cannot be reached:

      Greg [redact] on 5/10/2010 3:58 PM
            There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            <mail.example.org #5.5.0 smtp;550-Invalid recipient <greg.[redact]@sm[redact]ey.com>>

_Bounceback4________________________________________________________
Your message did not reach some or all of the intended recipients.

      Subject:       Meeting reminder
      Sent:      5/10/2010 3:57 PM

The following recipient(s) cannot be reached:

      Jason [redact] on 5/10/2010 3:58 PM
            Unable to deliver the message because the originator prohibited redirection to an alternate recipient or the message was encrypted
            <mail.example.org #5.7.3 smtp;530 5.7.3 Client was not authenticated>

_Bounceback5________________________________________________________
Your message did not reach some or all of the intended recipients.

      Subject:       Meeting reminder
      Sent:      5/10/2010 3:57 PM

The following recipient(s) cannot be reached:

      Greg [redact] on 5/10/2010 3:58 PM
            There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            <mail.example.org #5.5.0 smtp;550 Requested action not taken: mailbox unavailable or not local>

Answer : Bouncebacks from distribution group

Just to clarify, if you mail the individual address, it works... Is that by selecting the contacts from the list, or entering the addresses soome other way?

If you expand the distribution group (by clicking the +sign next to the group) do the addresses work?

Some of the issues I could understand, but #5 where it says the mailbox does not exist may indicate something is wrong with the address itself.

The other idea I had was depending on the number of addresses you are trying to reach in the same domain, domain maol host might be cutting you off after a specific number, thinking that your server is sending spam.