I have in the past reported emails being rejected. I was told this was due to rogue users who get Slingshot domain blacklisted
Accidentally tried the slingshot email again tonight , rejection again:

----- The following addresses had permanent fatal errors ----- <abc@abcd.com>
(reason: 550 Connections from the host mx195.callplus.net.nz (202.180.66.195), originating from SenderBase Ne...eputation Score. See http://www.senderbase.org for more information or contact your IT support team.)

----- Transcript of session follows ----- ... while talking to rcdn-mx-01.cisco.com.:
>>> RCPT To:<abc@abcd.com>
<<< 550 Connections from the host mx195.callplus.net.nz (202.180.66.195), originating from SenderBase Network Owner ID: 1565790, are being rejected due to a low SenderBase Reputation Score. See http://www.senderbase.org for more information or contact your IT support team.
550 5.1.1 User unknown
>>> DATA
<<< 503 #5.5.1 RCPT first

The page  http://www.senderbase.org/lookup/ip?search_string=202.180.66.195 seems to indicates that quite a few addresses or pool of addresses have been blacklisted. 

Slingshot, can you do something about it? you should be able to identify the accounts which have been compromised and are sending spam. Contact the users, block them if necessary.

This is getting annoying.