hMailServer 6.2.1 — B4 Deliverability & SMTP standards

This release completes Track B4: deliverability and modern SMTP standards.

New in 6.2.1

    PIPELINING (RFC 2920) advertised in EHLO.
    SMTPUTF8 / EAI (RFC 6531/6532): internationalized addresses accepted and relayed.
    ENHANCEDSTATUSCODES (RFC 2034): x.y.z codes on ESMTP replies.
    DSN (RFC 3461/3464): RET/ENVID/NOTIFY/ORCPT accepted; per-recipient NOTIFY honored (NOTIFY=NEVER suppresses failure DSNs).
    SRS (Sender Rewriting Scheme): HMAC-signed, reversible envelope rewrite on forwarding for SPF alignment; bounces decoded back to the original sender. Opt-in via [Settings] SRSEnabled/SRSSecret.
    Per-IP and per-destination SMTP rate shaping via [Settings] MaxSubmissionsPerIPPerMinute / MaxOutboundPerDestinationPerMinute (both default off).

Fixes

    Unicode::WideToMultiByte returned a trailing NUL that could corrupt DPAPI-protected stored secrets; now trimmed to the exact byte length.

All new features default to off / back-compatible. Builds clean (0 warnings, 0 errors); SMTP regression suite and in-server self-tests green.