Here is what happens When you send an e-mail:

1 - Your PC first sends it to the Web Builder Express mail server. 
2 - The Web Builder Express mail server tries to deliver it to the mail server of the recipient. 
3 - The recipient picks up the e-mail from their own mail server

Usually the transmission of the e-mail from step 2 to step 3 happens right away.  However, in some cases, the Web Builder Express mail server tried to send the message for 24 hours, but the recipient's mail server did not accept the message during that time. 

It could be that the recipient's mail server was down, or that e-mail account was closed.  After 24 hours, the Web Builder Express mail server gives up and sends you the message below.

To address this issue. you might want to try sending your e-mail again.  Perhaps the recipient's mail server was having temporary problems.  Or, check in with the recipient to see if they are having any problems with their mail service.