I have switched from zimbra (community) to MailInABox (https://mailinabox.email) and can say that a cheap five euros per month ssd-shared-4-virtual-cpu can manage as much as 1000nds of mails per day in and out for dozens of users with ease. So far I had not issues with spam classification, also due using a .de and .net email domain. (See stories about the infamous .xyz domains for reference) I tried Modoboa as well, but got stuck with the (lets encrypt) cerificate renewal process at that time - might be I did something wrong, don't know. Now everything runs automatic and smooth.