Gmail
Gmail is forgiving about layout and merciless about size. It also proxies every image, which changes how a dynamic banner has to work.
Gmail truncates a message and hides the rest behind “View entire message”. Our linter flags any signature approaching that.
Gmail caches images on its own servers, which is why our banner URLs carry a one-hour cache header rather than a long one.
Gmail strips @font-face, so we only offer stacks that are already on the machine.
The signature is copied as rich text, so it lands in Gmail settings intact rather than as raw HTML.
No account, no password. They open their link, check their details, press Copy signature, and follow the Gmail 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.