When the agent tests an app, including the auth pages, it sends real emails (e.g. signup confirmation). It often uses address such as name@test.com, or random ones associated with the app, like name@seller.test.
My app is using SMTP to send emails through my account, and I use AWS/SES to deliver them. And because these are dead mailboxes, I get bouncebacks through SES, which will harm my email reputation and lead to potential blocks in future.
Other people may use different methods to send emails from their apps (but still via SMTP or their own email accounts), and this issue will still affect them and lead to potential email account blocking.
Please look at ways of making the agent and test tool use real email addresses that are specifically setup for handling tests. eg. app-name-user-id-1@replit-test.com