* fix: support SMTP STARTTLS mode and NTLM auth
Add explicit SMTP STARTTLS configuration for 587-style connections and keep SSL/TLS as the implicit TLS mode.
Prefer PLAIN when advertised, keep LOGIN compatibility, and add NTLM as a fallback for Exchange SMTP servers that require it after STARTTLS.
* fix: respect explicit SMTP encryption mode
* fix: preserve SMTP TLS compatibility
