All the test email addresses are valid and can receive email messages. So, the message must not bounce back because the seed list has invalid email accounts.
If some of mailbox providers blocked your message with a bounce, read the bounce error description for more information. The message can be blocked because of the content, bad IP or domain reputation, or a lot of user complaints the mailbox provider saw for your emails in the past.
If all the messages bounced after you had sent them to the seed list, verify your sending process. It is not recommended to put all the test email addresses to the To, CC or BCC field. Gmail can reject the message if you try to send through the Gmail’s SMTP server using a large group of email addresses in the To, CC or BCC field. Create a group instead and send the message to a group email address.
Or you can consider a testing mode via one proxy address if it works for you.