Email and SMS campaigns, audience segmentation, and what happened after a send.
A segment is not a fixed list
A segment is a set of conditions, not a list of people. Recipients are recalculated every time a campaign sends.
So the number of recipients can change between building a campaign and sending it — and that is correct, not a fault. Someone who did not match yesterday but matches today receives today's send.
Audiences come from three places
A campaign can include customers, contacts and leads, each selectable independently. If the recipient count is lower than you expected, check which of the three are switched on before anything else.
The unsubscribe list always applies
Anyone on the suppression list is excluded from every send, whatever segment they fall into. This happens inside the send path itself rather than as an optional filter someone can forget.
So when a recipient is in the segment but did not receive the message, this list is the first place to look.
Statistics are derived from real state
Sent, opened and clicked counts come from per-recipient events, not from a counter written once at send time.
The consequence is that the numbers are not complete the moment you press send and keep rising afterwards. Open tracking also under-counts by nature — many mail clients never load the image, and that open is never recorded.