Check that Return email address for auto-generated emails contains a
valid e-mail address. (Some servers may require outgoing emails to be
sent from an email address on the domain that EE is running on. If
debugging shows a 553 error, make certain the return address complies
with this requirement.)
Try a different Email Protocol.
Attempt to send an email via
Developer ‣ Utilities ‣ Communicate. If that
fails it will output an error that will assist with troubleshooting the
problem.
If no error message is received it means that EE is successfully opening a
socket to the email server, writing the email headers and data to the mail
queue, and closing the connection. In this case, the next step is to contact
the hosting provider or server administrator for further troubleshooting.