Gmail rejects emails without a valid Message-ID header (RFC 5322).
Add Message-ID and Date headers to all outgoing emails.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Go's smtp.PlainAuth refuses to send credentials when the hostname
doesn't match the TLS cert (internal Docker hostname 'stalwart' vs
cert CN 'localhost'). Replace with custom LOGIN auth that works with
any SMTP server. Add detailed error logging at each SMTP stage.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Add SMTP_INSECURE_TLS env var to skip certificate verification for
internal Docker-network SMTP connections. Stalwart's self-signed cert
uses 'localhost' as CN which doesn't match the 'stalwart' hostname.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>