correct email rejected by Free ToastHost server

I received the following email bounce message using the correct email in the Membership Management section of our club website. I have masked the member's email for privacy. A separate email sent to this member through Outlook went through to the person just fine. Please advise. Fran

Your email message to xxxxxxxxx.com could not be delivered and was returned to the FreeToastHost server. Details are as follows:

Recipient Email Address: xxxxxxxxxx.com
Email Subject: RE: Recap 3/2/21 and Bell Tower Toastmasters Club Meeting Agenda for
Date Sent: Tue, 2 Mar 2021 20:59:38 +0000

Diagnostic Code: 550 5.7.1 Command rejected
Delivery Status: 5.7.1 securityerror
Date Last Attempted: Tue, 2 Mar 2021 21:15:33 GMT

NOTE: This email address may be invalid or non-working and should be changed, deleted, or updated to allow delivery.
If you have any questions about this, please visit us at support.toastmastersclubs.org

We need to be able to identify the member (unique first or last name) to investigate.

Lili Engelhardt
lili@engelhardt.com

Her email service rejected the email for security reasons, did it have any attachments or links?

The email contained 2 screenshots and the FTH link for our club agenda

See the complete meeting agenda at:
belltower.toastmastersclubs.org/agenda-1530112.html Print Email Again

Hers was the only message to bounce.
Fran

She would have to ask her service, we have no control over what various email services do.