Mailserver setup to manage diskspace and make it flexible

I have a mailserver doing all the work since years. With all the work I mean running Postfix, Dovecot, Clamav, Spamassassin. There are about 30 virtual servers (domains) on this mailserver. To tackle local diskspace (70% is in use) I can setup a second mailserver and start creating (for new customers) new virtual servers (domains) on that second server. The second server has in this setup nothing to do with the first server.

But, is there a better way to manage disk space? For example: use the first server as a gateway for all the domains and tell postfix/dovecot to store/read the mails for certain domains on server1 and other domains on server2.

If that is possible, are the login credentials (smtp and imap) stored on the gateway server or do i have to make users/domains on every server where the mail is stored?

Alain Cox