Corporate mail server setup with antispam and deliverability

Our company is looking for an experienced system administrator to fully configure our corporate mail server. Right now our messages keep landing in the spam folder, some emails never reach their recipients at all, and staff are losing track of important conversations. We want this fixed properly and for good, not with a temporary workaround. The scope covers deploying and tuning a mail server for several of the organisation's internal domains, creating mailboxes and distribution lists, and configuring cryptographic message signing according to the SPF, DKIM and DMARC standards so that our outgoing mail passes authentication at major providers and is no longer flagged as junk. We also need solid antispam on the inbound side: content filters, greylisting, sender reputation checks and protection against phishing attempts. The headline goal is high deliverability — letters must arrive in the inbox rather than the spam tray. Please describe your relevant experience, propose an architecture, give a realistic timeline and prepare a short guide for our team. Backups of mail data and queue monitoring would be a welcome bonus, along with a brief safety briefing for our employees.