maik 0.2.0

pub const OK: &[u8] = b"250 OK\r\n";
pub const AUTH_SUCCESS: &[u8] = b"235 2.7.0 Authentication successful\r\n";
pub const AUTH_INVALID_CREDENTIALS: &[u8] = b"535 5.7.8 Authentication credentials invalid\r\n";
pub const AUTH_UNSUPPORTED_METHOD: &[u8] = b"504 5.5.4 Authentication method not supported\r\n";
pub const BAD_MAILBOX_SYNTAX: &[u8] = b"553 Mailbox syntax incorrect\r\n";
pub const NO_MAIL_TRANSACTION: &[u8] = b"503 No mail transaction in progress\r\n";