Email Delivery Failure

For the past several years, I have sent email messages from katrina.martinez@prc.gov to the following PBGC Toastmasters club mailing list addresses:
members-7530@toastmastersclubs.org
prospects-7530@toastmastersclubs.org
friends-7530@toastmastersclubs.org

The past 2 times I sent the emails (yesterday and early August) my emails were not sent, and I received the attached Delivery Warning and Delivery Failure messages for each mailing list address.  I had another person outside PRC.gov send the emails, which were delivered without issue.  My agency IT Department (for prc.gov) said that based on the attached emails I received, the receiving system (FreeToastHost) was unable to process my emails.  Could you please help me determine why my emails were not delivered and how to fix the issue?  Thank you in advance for your help.

try it now.

Thank you for your help! I'd prefer not to send an email to the addresses right now because someone else sent the message for me, which went through. I don't want mailing list subscribers to receive too many emails. I will try again on September 6 when I send the reminder for the next meeting. I will post here if I have any issues.

Thanks again for your help!

The email I sent yesterday to the club mailing list addresses was delivered to recipients. Thanks again for your help!