When implementing SMTP transport with self-signed certificates, most modern MTAs (Mail Transfer Agents) won't outright reject your emails solely based on certificate validation failures. However, you'll face significant deliverability penalties:

• Trust scoring reduction: Services like SpamAssassin apply negative points for self-signed certs (typically -0.5 to -1.5 points)
• TLS downgrade risk: Some receivers may fall back to plaintext if certificate validation fails
• Reputation impact: Repeated certificate warnings may trigger sender reputation monitoring systems

Here's how different MTA configurations handle self-signed certificates:

# Postfix configuration (main.cf) for self-signed certs
smtpd_tls_cert_file = /etc/ssl/certs/selfsigned.crt
smtpd_tls_key_file = /etc/ssl/private/selfsigned.key
smtpd_tls_security_level = may  # Not "encrypt" which requires valid cert

# Exim4 configuration for opportunistic TLS
tls_advertise_hosts = *
tls_certificate = /etc/exim4/cert.selfsigned.pem
tls_privatekey = /etc/exim4/key.selfsigned.pem

You can detect certificate validation issues through these log patterns:

# Common rejection patterns in mail logs
"TLS handshake failed: certificate verify failed"
"NOQUEUE: reject: 450 4.7.1 Certificate validation failed"
"Warning: remote server presented unverified certificate"

Instead of self-signed certificates or plaintext SMTP, consider:

1. Let's Encrypt certificates: Free, automated, and widely trusted
2. Commercial certificates: About $50/year for basic validation
3. DANE/TLSA records: DNSSEC-based certificate validation (requires DNSSEC)

# Automated Let's Encrypt setup for Postfix
certbot certonly --standalone -d mail.yourdomain.com \
  --preferred-challenges http \
  --agree-tos -m admin@yourdomain.com

Limited use cases where self-signed certificates could work:

• Internal mail servers between known hosts
• Development/testing environments
• Systems where you control both sending and receiving MTAs

When configuring SMTP transports, many developers face the dilemma: self-signed certificates or plaintext? Let's break down what actually happens when your MTA uses self-signed certs:

# Example Postfix configuration with self-signed cert
smtpd_tls_cert_file = /etc/ssl/certs/mailserver.pem
smtpd_tls_key_file = /etc/ssl/private/mailserver.key
smtpd_tls_security_level = may

Modern mail servers evaluate several factors when encountering self-signed certificates:

• Opportunistic TLS: Most MTAs will still accept the message but without strict validation
• Reputation Systems: Major providers (Gmail, Outlook) may apply slight reputation penalties
• DANE/DNSSEC: Advanced systems requiring proper certificate chains will reject

Based on real-world testing across major providers:

Surprisingly, some cases where no encryption performs better:

# Python SMTP example showing security level options
import smtplib

# Option 1: Self-signed
server = smtplib.SMTP_SSL('mail.example.com', 465, context=ssl.create_default_context())

# Option 2: Plaintext (only for internal networks)
server = smtplib.SMTP('localhost', 25)

The most balanced approach for public-facing mail servers:

1. Free certificates from Let's Encrypt
2. Automatic renewal via Certbot
3. Full chain validation compatibility

# Certbot command for mail server certificates
certbot certonly --standalone -d mail.example.com \
--preferred-challenges http \
--non-interactive --agree-tos \
--email admin@example.com

For developers deciding on SMTP security:

• Internal systems: Self-signed acceptable
• Public email: Use Let's Encrypt
• Critical systems: Consider commercial certificates
• Always test with CheckTLS