Have a client with on prem Exchange who is trying to email a user at Slignshot. Was able to email them fine until about 1pm yesterday.

Now they just get bounce backs but the bounce backs (generated by mda-1.slingshot.co.nz) don't contain any useful information as to why except 5.0.0 which is a fairly generic error. (Syntax error command not recognized.)

The headers show it passed from client to SMX (which was happy), and passed to S/S who bounced it back.

Diagnostic information for administrators:

Generating server: mda-1.slingshot.co.nz

#< #5.0.0> ##

Any ideas ?