The best way I have found to run an email server is to use it to receive emails only and to get your own emails out send them directly through a provider like A

The best way I have found to run an email server is to use it to receive emails only and to get your own emails out send them directly through a provider like Amazon SES. Now you can create all of the email accounts with as much space as you want but still get your emails delivered. It is not ideal but the only practical way to run a small email system.