Pause a campaign that is currently sending, setting its status to paused (3).

Only a campaign whose status is sending (2) can be paused. Pausing clears the send date on the campaign's most recent process row, which stops the sending engine from picking it up again; a campaign whose process has already finished everything it was given cannot be paused and returns succeeded: 0 with Process not found..

This endpoint reports business-rule failures in the response body rather than with an
error status: an unknown campaign, an automation ("series") campaign, a completed campaign and a
transition that is not allowed from the campaign's current status all return 200 with succeeded
set to 0. The message text is localized to the account language, so match on succeeded rather
than on the message.