Timeout when delivering to large group of aliases

[List]

I wanted to confirm the behavior we are experiencing at the moment when 
delivering messages to addresses aliased to thousands of local users.  
For example we have the address distgr...@domain.tld which is an alias 
to 3000 local users.  When our inbound spam filter connects to the 
Postfix server to relay a message to this user we are seeing a timeout 
after 60 seconds and the message gets deferred on the filter, but the 
message has actually been delivered to the alias and subsequently all 
the recipients.  The filter then retries the deferred message and we 
start having duplicate messages to the users.

Is it true that Postfix is waiting to send 250 OK back to the filter 
until all the recipients have had a copy of the message delivered to 
their inbox?  If so is there a more efficient way to go about delivering 
to many thousands of aliases?