Single email to guests@ has been sent out 10x now! It won't stop sending it out!

Yes, the email included one attachment — a blank Toastmasters membership application form.Initially, I tried sending the email to both mailing lists together:

• prospects-643436@toastmastersclubs.org
• members-643436@toastmastersclubs.org

The first attempt used both lists in the “To” field, and the second attempt used both lists in “CC,” but both attempts appeared to fail to send.After that, I sent separate emails to each group individually, and those appeared to send successfully.However, I later noticed that only the prospects group seems to be repeatedly receiving the same email. The members group does not appear to be affected. Some prospects reported receiving more than 20 duplicate emails.