After years of wrestling with Exchange alternatives, I've identified the key pain points that make administrators keep returning to Microsoft's solution. The critical factors boil down to:

# Core requirements checklist
REQUIREMENTS = {
  'protocol_support': ['MAPI', 'EWS', 'ActiveSync'],
  'auth_backends': ['LDAP', 'SQL', 'PAM'],
  'high_availability': True,
  'outlook_compatibility': 'Full feature parity',
  'resource_booking': ['rooms', 'equipment', 'vehicles']
}

Let me walk through the most promising options based on actual deployment experience:

1. Kopano Core (Formerly Zarafa)

This open-source solution offers remarkable Outlook compatibility through its proprietary MAPI implementation. A basic HA setup with PostgreSQL streaming replication:

# Kopano HA configuration snippet
server {
  listen 236;
  protocol = http;
  server_name mail1.example.com mail2.example.com;
  
  location / {
    proxy_pass http://kopano_gateway;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
  }
}

upstream kopano_gateway {
  server 10.0.0.1:236;
  server 10.0.0.2:236 backup;
}

2. Open-Xchange (OX)

While primarily web-based, OX provides solid Outlook integration through its OX App Suite connector. The resource booking system is particularly robust:

// OX API example for resource booking
POST /ajax/resources?action=book
Content-Type: application/json

{
  "resource_id": "conf_room_12",
  "subject": "Quarterly Review",
  "start": "2023-11-15T09:00:00Z",
  "end": "2023-11-15T11:00:00Z",
  "organizer": "user@domain.com",
  "attendees": ["user1@domain.com", "user2@domain.com"]
}

Based on production deployments, these factors make or break the transition:

• Outlook Profile Migration: Use XML config files to preconfigure clients
• Calendar Synchronization: Test recurrence patterns and timezones thoroughly
• Autodiscover Implementation: Essential for zero-touch Outlook setup

Recent tests on identical hardware (32vCPU, 64GB RAM, NVMe storage):

For organizations with existing Exchange infrastructure:

# Sample imapsync command for mailbox migration
imapsync \
  --host1 exchange.olddomain.com \
  --user1 mailbox@olddomain.com \
  --password1 "oldpass" \
  --host2 kopano.newdomain.com \
  --user2 mailbox@newdomain.com \
  --password2 "newpass" \
  --subscribe \
  --syncinternaldates \
  --addheader \
  --exclude "Deleted Items" \
  --exclude "Junk Email"

After years of administering email systems, I've learned one universal truth: users love Microsoft Outlook. The challenge is finding a Linux-compatible solution that delivers Exchange-like functionality without compromise. Here's what actually works in production environments.

• Authentication: Must integrate with existing LDAP/AD or SQL infrastructure
• Web Interface: Polished UI comparable to OWA (Outlook Web Access)
• High Availability: Proper clustering and failover capabilities
• Outlook Compatibility: MAPI or EWS protocol support is non-negotiable

1. Kopano (formerly Zarafa)

This German-engineered solution offers remarkable Outlook compatibility through its proprietary MAPI implementation. The web interface (WebApp) is surprisingly polished.

# Sample Kopano installation (Debian-based)
wget -qO - https://download.kopano.io/stable:/final/Debian_10/Release.key | apt-key add -
echo "deb https://download.kopano.io/stable:/final/Debian_10/ ./" > /etc/apt/sources.list.d/kopano.list
apt update && apt install kopano-server kopano-webapp

2. SOGo (Open Source)

While requiring more configuration, SOGo shines with its standards-compliant approach using ActiveSync and CalDAV/CardDAV.

# NGINX configuration for SOGo
location /SOGo {
    proxy_pass http://127.0.0.1:20000;
    proxy_redirect http://127.0.0.1:20000 default;
    # Enable WebSockets
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
}

The real test is Outlook integration. Kopano's MAPI provider creates the most seamless experience, while SOGo requires the OpenChange connector or native EWS support in newer Outlook versions.

For resource booking, both solutions support iCalendar-based room/resources management. Here's how to configure a conference room in SOGo:

# Sample SOGo resource definition
{
    "name": "Conference Room A",
    "email": "room-a@domain.com",
    "isGroup": false,
    "isResource": true,
    "delegates": ["admin@domain.com"]
}

For true redundancy, consider this PostgreSQL replication setup for SOGo:

# PostgreSQL streaming replication (primary)
wal_level = replica
max_wal_senders = 3
hot_standby = on

# On secondary server:
primary_conninfo = 'host=primary.db.server port=5432 user=replicator password=secret'

When moving from Exchange, tools like imapsync become essential:

imapsync --host1 exchange.server --user1 user@domain.com --password1 secret \
         --host2 new.linux.server --user2 user@domain.com --password2 secret \
         --syncinternaldates --addheader --subscribeall

For calendar migration, consider using the exchange2ical tool before importing into the new system.

After implementing both solutions in production, Kopano delivers the most "Exchange-like" experience out of the box, while SOGo offers greater flexibility for organizations willing to invest in configuration. Both solutions meet the core requirements when properly implemented.