Howto cache outgoing mails?

[Del]

I am looking for a way to cache outgoing mails on the Kolab server. The 
issue is the following: When I send large mails (e.g., through the Horde 
web-interface), the message window stays open (with no progress bar) 
until the entire message has left the Kolab-server. Preferably I would 
like the message window to close immediately once I press the "Send" 
button, having the Kolab-server caching the message until it is delivered.

Kontact essentially has this functionality built in by using the outbox, 
but it would be nice having it on the server too. I would guess it is a 
setting in Postfix, but I haven't found it, so very grateful for any help.

Cheers,
Del