Pressing Send Campaign does not send anything by itself – it creates one queued message per recipient with the chosen send date. A background job on the server then runs on a schedule and, on each run, takes up to 50 queued messages whose send time has arrived (oldest first) and hands them to the email provider. So a campaign of 1,000 recipients is delivered in batches over several runs; while that is happening the campaign sits in the Outbox tab with a Recipients column like 350/1000. A separate job re-checks each campaign's status (Scheduled → Outbox → Sent/Error) roughly once a minute, though the exact run frequency of both jobs is configured on the server rather than in the admin panel.

If a queued message fails to send, a single failure is already enough to move the campaign into the Error tab (its error text appears when you open the campaign). Sprwt keeps retrying a failed message on later runs, but after 3 failed attempts it stops retrying and the message is left stuck in the queue.