Email is an asynchronous "best effort" process. It's not an actual send/receive from you to the recipient but more like a game of telephone played between your ISP to server to server to server to the recipient's ISP and finally into their mailbox. Usually an email is received in a matter of seconds or minutes, but it's not unheard of to see delays of hours or even days depending on servers in the middle.
You can always see email we send immediately in the Emails tab of OwnerRez -- that shows all emails in or out of the system. If you check there, you don't have to wait for it to show up in your or the guest's inbox.
