Verifying email performance is more than the basic understanding of message flow. Outbound mail in the form of Simple Mail Transfer Protocol (SMTP) and inbound mail through MAPI or Microsoft's Graph API only parts of email systems to monitor, usually through pings or basic delivery confirmations. Often, once email is moved to Exchange Online, even basic visibility of mail flow and reliable delivery is lost. Many subsystems go into efficient email deliverability, especially once multiple email hygiene providers are added to the mix.