How to choose: answering the main questions
How many resources are needed for email?
Zimbra/Mailcow for 100 mailboxes: 4–6 cores, 16–24 GB RAM. Exchange is more resource-intensive: 8+ cores and 32–64 GB. The key is storage: email accumulates over years, so plan for at least 5 GB per mailbox annually.
SSD or HDD for storage?
Indexes and current database on SSD, attachments and archives on HDD. A hybrid setup provides fast search through correspondence without overpaying for terabytes of flash storage.
What about delivery?
Clean IP, correct SPF/DKIM/DMARC, and reverse DNS — we configure these during deployment. It is better to move outgoing mailings to a separate relay to avoid damaging the reputation of your main domain.