The raw MIME template in sendRawEmail produced a blank line between Content-Type and List-Unsubscribe whenever no custom headers were passed, because the ternary for custom headers expanded to an empty string surrounded by newlines.
Per RFC 5322 §2.1 a blank line terminates the header section, so List-Unsubscribe ended up as the first line of the body. Lenient clients (Gmail, Outlook) recover; strict clients (Thunderbird) do not, breaking one-click unsubscribe and degrading Gmail/Yahoo bulk-sender deliverability signals.
Collect optional headers (custom + List-Unsubscribe) into an array, filter empties, and append them to the Content-Type line with a single newline separator. No codepath can now produce a blank line inside the header block.
Matthias Endler