Message status classificators are defined as human-readable constants.

ok Message has been successfully delivered.
bad-mailbox Message recipient is non-existent, inactive or disabled.
blocked Delivery has been denied from Smaily (or any service).
expired Message delivery has taken more than maximum amount (2 days) of time.
inbox-full Mailbox quota overused by recipient.
message-size Message is too large for receiving email server.
routing-error Invalid (or non-existent) DNS entry or domain name.
signature-error Incorrectly configured SPF or invalid DKIM signature.
spam Message has been identified by the receiving email server as SPAM. Determined by the content or sender of message.
transit-error Hourly/daily sending limits exceeded, receiving system is under heavy load or undergoing maintenance.
unknown Message status unknown.