If memory serves EV uses the first limit it finds. You may want to suggest customers to remove the prohibit send\receive limits, because if for some reason they reach the prohibit limits Archiving can't occur. If the user can't send\receive neither can the mailbox be archived. Seen that support case a few to many times.:)
There is also this for 5.0 sp6:
Quota archiving and mailbox 'Prohibit send' value
It is now possible to perform archiving by quota in which the mailbox Prohibit send value is taken to be 100% quota. In order to do this, set the PreferQuotaNoSendLimit DWORD registry entry under the following registry key on the server hosting the mailbox Archiving Service:
HKEY_LOCAL_MACHINE
\SOFTWARE
\KVS
\Enterprise Vault
\Agents
Set PreferQuotaNoSendLimit to 1 to archive with the mailbox Prohibit send value as 100% of quota.