4.X.X Persistent transient failure
The mail server encountered a temporary failure. If the command is repeated without any change, it might be completed. Mail servers can use temporary failures like this to keep untrusted senders at bay.
5.X.X permanent error
The mail server has encounted a permanent error. These errors will result in the SMTP connection being dropped. Re-sending will usually produce the same result.
SMTP Field Manual
The SMTP field manual is a collection of raw SMTP server responses from major email service providers and spam filter services. The data is open source so anyone make contributions. Even you! See something interesting from an email service provider or simply have a suggestion? Let us know.