[codex] fix SMTP sender validation for digit-leading domains

[Phillipxh]

Summary

• route SMTP sends through a small LenientSmtphordeTransport wrapper
• disable Horde's strict From header validation for sender addresses so domains like 180grad-kiel.de are accepted
• add a regression test for digit-leading domains and update the SMTP factory test to cover the new transport

Why

Horde_Mail_Transport::prepareHeaders() uses Horde_Mail_Rfc822 with validate => true for the From header. That rejects valid domains whose labels start with digits, which breaks message sending for addresses like user@180grad-kiel.de even though RFC 1123 permits them.

This keeps the workaround local to the SMTP transport used by Mail instead of patching vendor code in place.

Validation

• Added a unit regression test for From: Test User <user@180grad-kiel.de>
• Ran git diff --check
• Could not run PHPUnit locally in this environment because php is not installed

Closes #12992