This is due to a combination of certain settings, primarily surrounding email security and privacy, JMU emails (addressed from JMU) sent to certain domains (IE., AOL, Yahoo, Hotmail), may experience delays.
These emails will be delivered, but depending on the domain owner, it could take upwards of 4-12 hours.
Specific emails that come from "No-Reply@jmu.edu" or JMU Listserv addresses are more susceptible.
The JMU IT Department is currently working towards a resolution for this issue, but there is not a specific time frame at this moment.
Symptoms
Email from: Mail Delivery Subsystem
Message: This is a warning message only. You do not need to resend your message.
Warning: message still undelivered after 4 hours. Will keep trying until message is 5 days old.
Keep getting an error message on emails that I'm sending to a specific email address