The email addresses have expired.
Create auto expiring inboxes using MailSlurp disposable mailboxes. Email accounts that expire and self destruct.
MailSlurp inboxes can be set to expire at a given time. The server periodically checks inboxes to see if they have expired and archives those that have.
Expired inboxes can no longer sender or receive emails. You can however view old emails using the inbox's ID.
If you have a free account a default expiration date is used when creating inboxes. Paid accounts can set whatever expiration dates your desire or leave the property blank for permanent inboxes by default.
Working with expired inboxes
See the developer documentation to list and fetch expired inbox details.