If you want a certain message to be sent to users only once without any further repetitions, please add the following conditions to the user segment:
|Condition type||Event count|
*The Event Type may vary in case you are referring not to email campaign but any other campaign type.
|Value||Your current campaign ID|
|from beginning of time to dispatch|
This condition ensures that the message will be sent only to those users who have received 0 emails (namely, never received an email) from the current campaign since the campaign has been launched.