Kæmpe SPAM problemer

Jeg blev i dag vækket med at vores system ikke modtog og sendte mails. Da jeg kom derind havde vi flere 1.000 queues i Exchange og ca. 78.000 messages lø i vores kø.

Det jeg gjorde for at løse problemet her og nu var at slette hele vores kø.

Har i ca. 14 dages tid døjet med fejlmeddelelsen:

Event ID: 7010
This is an SMTP protocol log for virtual server ID 1, connection #540. The client at "202.75.56.212" sent a "rcpt" command, and the SMTP server responded with "550 5.7.1 Unable to relay for hidesnow7423@yahoo.com.tw  ". The full command sent was "rcpt TO: <hidesnow7423@yahoo.com.tw>".  This will probably cause the connection to fail.

og

Event ID: 7004
This is an SMTP protocol error log for virtual server ID 1, connection #138. The remote host "12.181.131.5", responded to the SMTP command "xexch50" with "504 Need to authenticate first  ". The full command sent was "XEXCH50 2356 2  ".  This will probably cause the connection to fail.

Hvad er det der sker? Er det DoS? Hvordan sikre vi os at det ikke sker igen?

P.S.:
Jeg har været inde og lege med IMF (Intellignet Message Filtering) og SIF (Sender ID Filtering) begger er sat til Reject.
Yderligere ser Relay Resctriction sådan ud:

Select which computer may relay through this virtual server:
Only the list below (is checked)

Allow all computers which successfully authenticate to relay, regardless of the list above. (is checked)