Exchange 2007 Hub Transport Server cannot see all DCs

We have four Exchange 2007 servers:
   MELEX01 and MELEX02 (Shared Storage clustered mailbox servers)
   MELEX03 - Hub Transport and Client Access
   MELEX04 - Unified Messaging

MELEX01, 02 and 04 ADACCESS reports all the domain controllers:
   In-site:
    meldc01.melb.avtech.com.au      CDG 1 7 7 1 0 1 1 7 1
    mel2k02.avtech.com.au      CDG 1 7 7 1 0 1 1 7 1
    MELBU01.melb.avtech.com.au      CDG 1 7 7 1 0 1 1 7 1
  Out-of-site:
   SHADC01.melb.avtech.com.au      CDG 1 7 7 1 0 1 1 7 1
   HCMCDC01.melb.avtech.com.au      CDG 1 7 7 1 0 1 1 7 1

MELEX03 however, only shows two and doesn't show MELDC01 as a GC:
  In-site:
   melbu01.melb.avtech.com.au      CDG 1 7 7 1 0 1 1 7 1
   meldc01.melb.avtech.com.au      C-- 1 6 7 0 0 1 1 7 1
 Out-of-site:
 
All servers (in site) are on the same subnet and are not firewalled

DCDiag on MELDC01 doesn't reveal anything suspicious.

Site info:
   MEL2K02 is a DC for the top level domain avtech.com.au and holds the schema master role
   MELBU01 is a DC for melb.avtech.com.au and is a GC
   MELDC01 is a DC for melb.avtech.com.au and holds all FSMO roles and is a DC

We are getting ready to decomission MELBU01 so it concerns me that the exchange server MELEX03 cannot see any other GCs.

Any suggestions?