Apple Mail
Apple Mail is the most capable renderer of the lot, and it has one setting that quietly breaks every signature people paste into it.
On by default in Apple Mail preferences, and it strips your typography. Our install steps tell people to untick it, which most guides omit.
Apple Mail leaves explicitly coloured text alone but inverts the rest, so our layouts carry their own light background.
Apple Mail honours @font-face — but Outlook and Gmail do not, so using one would mean two different signatures in the wild.
Every image ships at twice its display size, which matters more on a Mac than anywhere else.
No account, no password. They open their link, check their details, press Copy signature, and follow the Apple Mail steps on the same page. When the brand changes, the link still works — it never expires.
And the campaign banner inside it can change after they have pasted it, because the signature points at a permanent URL rather than embedding the artwork.