A 524 can also be caused by your origin server being overloaded- if you see this error it's a good idea to check your server's available resources, including CPU and RAM, and overall traffic levels. If your server has a high CPU load, or is low on memory, this could indicate a resource problem.
If you're unsure about the health of your server it's a good idea to contact your hosting provider and ask them to investigate.
E-Mail Server Problem -
Still not able to get my e-mails to go out to Subscribers -
After hitting the "Send" block, e-mails are Not getting Sent.