When Microsoft's email services (Hotmail/Outlook) reject your properly configured emails despite SPF/DKIM/rDNS being correctly set up, it typically points to deeper infrastructure and reputation issues. Here's what I've discovered through extensive testing:

First, let's verify your current Postfix configuration. The headers show correct authentication, but we need to check the actual implementation:

# Sample Postfix main.cf configuration
smtpd_banner = $myhostname ESMTP $mail_name
biff = no
append_dot_mydomain = no
readme_directory = no

# TLS parameters
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache

# Authentication
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
smtpd_sasl_local_domain = $myhostname
smtpd_client_restrictions = permit_sasl_authenticated,reject

Microsoft uses a complex sender reputation system that considers:

• IP history (even if not blacklisted)
• Domain age and traffic patterns
• User engagement metrics (if recipients mark as spam)
• Volume spikes (new IPs sending bulk emails get scrutinized)

For new IPs, implement this warmup schedule over 4-6 weeks:

# Warmup schedule script example (Python)
import smtplib
from email.mime.text import MIMEText

def send_warmup_emails(recipients, daily_limit):
    msg = MIMEText("Test content")
    msg['Subject'] = 'Testing email deliverability'
    msg['From'] = 'valid@yourdomain.com'
    
    server = smtplib.SMTP('localhost')
    for i, recipient in enumerate(recipients[:daily_limit]):
        msg['To'] = recipient
        try:
            server.sendmail(msg['From'], recipient, msg.as_string())
            print(f"Sent to {recipient}")
        except Exception as e:
            print(f"Failed to send to {recipient}: {str(e)}")
    server.quit()

# Gradual increase
week1_recipients = [...]  # 10-20 known engaged addresses
week2_recipients = [...]  # 50-100 addresses
send_warmup_emails(week1_recipients, 10)

Beyond standard protocols, Microsoft recommends:

1. Register with their Smart Network Data Services (SNDS)
2. Implement DMARC policies
3. Maintain consistent reverse DNS (PTR records should match forward DNS)

Essential tools for troubleshooting:

# Command line checks
dig example.net txt  # Verify SPF
dig mail._domainkey.example.net txt  # Verify DKIM
dig -x your.ip.address.here  # Verify rDNS

# Microsoft-specific tools
curl -v "https://sendersupport.olc.protection.outlook.com/snds/.../auto"

Remember that even with perfect technical configuration, Microsoft's algorithms may still require time to establish trust for new sending infrastructure.

After setting up Postfix+Dovecot with all recommended email authentication protocols, I found Hotmail/Outlook stubbornly marking legitimate emails as spam. Here's what I discovered through painful trial and error.

While the headers show passing SPF/DKIM:

Authentication-Results: hotmail.com; 
    sender-id=pass (sender IP is 66.85.140.94) 
    header.from=info@example.net; 
    dkim=pass header.d=example.net; 
    x-hmca=pass

Hotmail employs additional secret sauce filtering beyond standard protocols. Their SmartScreen filter analyzes behavioral patterns we need to accommodate.

Microsoft requires gradual email volume increases for new sending IPs. Start with:

# Postfix rate limiting for warm-up
smtpd_client_message_rate_limit = 10/60s
smtpd_client_connection_rate_limit = 5/60s

Recommended warm-up schedule:

• Day 1-3: 50 emails/day (personal correspondence only)
• Day 4-7: 100 emails/day (add newsletter subscriptions)
• Week 2: 500 emails/day
• Week 3: 1000 emails/day

While not mentioned in original setup, DMARC provides crucial reputation signals:

# Example DMARC DNS record
_dmarc.example.net. IN TXT "v=DMARC1; p=none; pct=100; 
    rua=mailto:dmarc-reports@example.net; 
    ruf=mailto:dmarc-forensics@example.net"

Hotmail specifically filters these content patterns:

# Problematic phrases to avoid:
- "Click here"
- "Limited time offer"
- "Dear valued customer"
- Exclamation points!!!

Instead use:

# Better alternatives:
- "View details at [domain]"
- "New opportunity available until [date]"
- "Hello [firstname]"
- Periods. Like this.

Essential Postfix additions for Microsoft delivery:

# In main.cf
smtp_header_checks = pcre:/etc/postfix/header_checks
disable_vrfy_command = yes
strict_rfc821_envelopes = yes

# header_checks file
/^Subject:/ REPLACE Subject: Your Actual Subject
/^User-Agent:/ IGNORE

Essential resources beyond standard blacklist checks:

• Microsoft SNDS (Smart Network Data Services)
• JMRP (Junk Mail Reporting Program) enrollment
• Outlook.com Postmaster portal

For serious senders, these steps significantly improve delivery:

1. Register at Microsoft Sender Center
2. Authenticate your domain through Microsoft 365
3. Submit IPs for whitelisting consideration