When emails are sent they are either accepted by the recipient mail server (the remote server) or a failure is reported. If the failure is "permanent" the reports will show a bounce. Email sending isn't retried.
If the remote server reports an error using an error code that classifies it as a "transient" failure then email sending is retried automatically over a 48hour period up to 10 times.
This is handled by our SMTP server, not visible from the interface. This means retries cost no credits, and messages in the retry queue are indistinguishable from successfully sent messages until the retries are finished and a bounce is reported if the retries were unsuccessful.