Are you sure it's the Mozilla Postmaster telling you this? What's the exact message? That quota message is getting sent by your email service provider's server, not Mozilla, unless Mozilla is your email service provider (Mozilla doesn't offer mail service as far as I know). You could be deleting several hundred messages, but they may not be sizeable enough to free up significant space in your disk quota. I suggest you contact your email service provider and have them check your quota as provisioned on their server. Log in to your hosting account and check what the actual disk usage is.