Fix Odoo 17 Email Issues

I run Odoo 17 Community and have hit a road-block with the Marketing Email module: every campaign I send goes out with a completely blank subject line. Alongside that, some templates lose their formatting once they reach the inbox.

I need an Odoo specialist who can:
• trace why the subject header is not being populated in mass-mailings and restore it for both ad-hoc and automated campaigns,
• correct the template rendering so the emails look exactly as they do in the Odoo editor,
• leave clear, step-by-step notes so I can repeat the fix on staging and future upgrades.

If the root cause touches SMTP settings, list IDs, QWeb/Jinja templates, or any recent module updates, please patch them and test thoroughly. A short call or screen-share at the end to walk me through what you changed will close the job.