How do expired certificates cause bounces?

Expired certificates break TLS authentication:

Certificate validity:

Certificates have defined validity period. After expiration, they are no longer trusted. Clients refuse to establish TLS connection.

In email delivery:

STARTTLS upgrade fails. Connection may be rejected or fall back to plaintext. Strict TLS policies cause hard failure.

Who is affected:

Sending server certificate: Affects outbound authentication. Receiving server certificate: Affects inbound connections. Both need valid certificates.

Typical errors:

"Certificate has expired." "SSL_ERROR_EXPIRED_CERT." 454 TLS not available.

Resolution:

Renew certificate before expiration. Implement automated renewal (Let's Encrypt). Monitor certificate expiration dates.

Expired certificates are outdated credentials. They no longer prove your identity.