Unable to deliver email address for specific address when emailing agendas

When I email the meeting agenda the following individual Leah Cole (invalid/no email address) appears. I logged in as an administrator and noticed the email was removed when I re-entered her email address leah@k-lakcorp.com and saved. I was able to email the agenda with no issues. The next time I emailed the agenda the message appeared again with invalid/no email address). This is a valid email address. For now I forwarded the email to her email address.
I would like to not have to do this workaround every time. Can you tell me what the issue is and how it can be fixed?

She needs to whitelist the email addresses server@toastmastersclubs.org and agenda@toastmastersclubs.org. There is nothing we can do if her email service blocks the emails.

here is the error

Your email message to leah@k-lakcorp.com could not be delivered
and was returned to the FreeToastHost server. Details are as follows:

Recipient Email Address: leah@k-lakcorp.com
Email Subject: Caesar Rodney Toastmasters 2020-03-10 Meeting Notification:
Date Sent: Wed, 26 Feb 2020 13:37:15 +0000

Diagnostic Code: 554 5.7.1 The message from () with the subject of (Caesar Rodney Toastmasters 2020-03-10 Meeting Notification: Attendance) matches a profile the Internet community may consider spam. Please revise your message before resending.
Delivery Status: 5.7.1 securityerror
Date Last Attempted: Wed, 26 Feb 2020 13:37:16 GMT