Response codes starting with number four mark a temporary problem. Email server will try to send newsletters again a couple of times.

Some examples:

422 The recipient's mailbox has exceeded its storage limit or the size of the message exceeds the recipient’s size limits for incoming email.

431 Not enough space on the disk, or an "out of memory" condition due to a file overload.

441 The recipient's server is not responding.

444 The same as with 544 - transferring was (temporarily) unsuccessful, with 544 it's permanently.

450 Requested action was not taken – The mailbox was unavailable at the remote end (possible reason - it's due to server maintenance server is offline).

471 An error of your mail server.

Response codes starting with number five mark a problem that concerns the recipient's email address.

Some examples:

500 A syntax error: the server couldn't recognize the command. 

510 Bad email address.  

511 The same as with 510 - bad email address. 

512 A DNS error: the host server for the recipient's domain name cannot be found - probably the email address is faulty or doesn't exist.

521 Domain does not accept mail. The recipient mail server does not actually accept and deliver email itself. 

522 The same as with 521 - domain does not accept mail. The recipient's mailbox has been full for a long time and probably the email does not exist anymore.

523 The same as with 521 and 522 - domain does not accept mail. The total size of your mailing exceeds the recipient server's limits.

532 Server does not accept mail (probably because the recipient's server is overloaded or due to server maintenance or sender server's bad reputation (e.g IronPort can set those restrictions)).

535 Several servers use the same IP address. An authentication is required. 

544 The recipient's server transferring was unsuccessful (probably due to an incorrectly set mailbox). 

546 Looping problem. A contact has the same email address as an Active Directory user. One user is probably using an Alternate Recipient with the same email address as a contact.

550 Requested actions not taken as the mailbox is unavailable. The recipient email address does not exist.

571 Permissions problem. For some reason the sender is not allowed to email this account. Perhaps an anonymous user is trying to send mail to a distribution list.

Other response codes:

Subscriber hasn't received any error codes.

200 Delivery successful.

1001 Subscriber unsubscribed in the latest campaign.

1002 Subscriber reported the latest campaign as spam. Is automatically unsubscribed.