SMTP Error 446 : The maximum hop count was exceeded for your message. The most likely cause of this error status code is that your message is looping internally on your server, internally between two of your organisation’s servers, or, sometimes, looping between your server and the recipient’s server.

SMTP Error 442 : Your server started delivering the message but the connection was broken during transmission. This may be an unusual transient error – however, if it keeps happening you should investigate possible problems with your server’s network card, your Internet routers, processes hogging the resources of your server, and anything else which could result in a network connection being broken.

SMTP Error 441 : This is an error emanating from your server indicating that the recipient’s server is not responding. Your server will automatically try again a number of times – how many depends on how your server has been configured.

SMTP Error 432 : This is an SMTP status response specific to Microsoft Exchange Server. It indicates that the recipient’s mail queue on their Exchange Server has been stopped (frozen), probably while the Network Administrator troubleshoots some problem.

SMTP Error 431 : The recipient’s mail server is experiencing a Disk Full error condition, or an Out of Memory (too many file handles) error condition (Microsoft Exchange).