Once the emails are processing to be sent,  you cannot stop the emails. You can always pause your campaign before the emails are processed to be sent as described here; How do I pause a Campaign

If you really need it to stop. You can contact [email protected] for us to temporarily disconnect your mailbox. However, we don't really recommend this alternative for once we  disconnect your mailbox. the reply detection will no longer work and any emails connected to this email will not send. 

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