Although SMTP is a very reliable service, there is no SLA on its actual delivery. There may be many handlers in any email that is sent from any system.
In light of this, CopperEgg strongly recommends that you enable other notification methods as part of any alert that, where there is a priority of knowing the alert has fired.
CopperEgg offers pagerduty, SMS, mobile push, hipchat, Slack, Opsgenie, twitter dm and webhook as additional notification mechanisms.