How to stop Sys Admin from Bouncing messages - Outlook 2010 / Gmail

I have a .cvs file generated from Excel of 70 contacts including their emails. I think I have successfully imported them into a separate Contacts folder that I created (BDHG Contacts). They all look mapped correctly and properly formatted when I scroll through the BDGH Contacts view. I then created a Distribution List using all 70 email contacts. When I send to the list I usually get about 12 email notifications from the Sys Admin. They all say:
"The following recipient(s) cannot be reached:

SMITH on
The recipient has been deleted or has no e-mail address."
FYI I use Gmail Imap. I know the email addresses are good ones. Any idea of how to fix this?

Thanks much!

The email addresses work when sending individually. The "bounced" recipients email services are with different providers.

Microsoft told me it would be fixed if I removed my Gmail account from Outlook, re-started Outlook, and then re-installed Gmail. That did not work.

Bree said:

Can you address it using bcc: ? That way each recipient's mail server would only see it as being addressed to a single recipient, not the entire mailing list. May help to stop their Sys Admin rejecting it as spam.

Thanks for the suggestion. I will give this a try to see what happens. I don't think spam rejection is the issue, though, because I have 100 recipient DLs that I regularly send to without any issues. OH! Come to think of it I DO use Bcc with the other DLs.