Documentation
A pasted signature is frozen HTML in someone else's mail client. The banner is the part that isn't: swap a campaign and it changes inside every signature already installed.
A slot is a fixed position in the signature with a permanent image URL and a permanent click-through URL. The signature embeds those URLs once; the server decides what they serve. A campaign is a banner image, a destination link and a date window pointed at a slot — whichever active campaign has the highest priority is what every installed signature shows, within the hour.
Give a campaign a start and end date and it runs itself: the moment the window opens the banner appears in everyone's signature, and the moment it closes the next campaign (or nothing) takes over. Overlapping campaigns are resolved by priority — highest wins. No re-rollout, no asking anyone to re-paste.
A slot with nothing to show can either serve a fallback image or collapse to nothing — set per slot. Collapsing is the default, so an empty slot never leaves a broken-image icon in anyone's email.
Because the click-through is a redirect we serve, every campaign gets click and impression counts for free — no pixel, no cookies, nothing added to the signature. The numbers sit on the campaign row.
Banner images carry a one-hour cache header, which is the deal that makes this work: mail clients and Gmail's image proxy are allowed to keep the image for an hour, so a campaign swap reaches installed signatures within that hour rather than instantly. Nothing about a slot may be cached longer, and nothing needs to be.
