Impression pixel
A tiny invisible image used to estimate how often a signature is seen.
An impression pixel is a 1×1 transparent image in the signature. Each time a mail client downloads it, the server counts a view.
The count is an estimate. Image proxies cache images, some clients block them, and privacy features may load images in advance. Treat impressions as a trend, not an exact number.
Sendoff aggregates impressions per day and uses them only to estimate click-through rates for banners.