I ran into this issue when I was generating emails as part of a workflow process. It would create the email task, but none of them would actually send and when I looked at the task, it would say "This message has not yet been submitted for delivery" at the top of the task in a yellow bar. No explanation, no nothing.
The first thing I did was make sure that the CRM email router was configured accurately on the CRM server.
FIRST, I had my EMAIL-OUT configuration setup against our corporate SMTP server.
SECOND, I applied my EMAIL-OUT configuration to my CRM instance.
THIRD, I tested my EMAIL-OUT configuration.
FOURTH, I manually created an email task and made sure it sent successfully.
That’s what helped me to realize that the issue was with the emails being generated by my workflow. It turns out that there is a little "bug/feature" in the CRM system. When you create an email task in a workflow process, you have to make sure that you specify a value for No. Delivery Attempts. If you don’t, the subsequent value will be NULL, and the email router will ignore it. Therefore, the email will never be sent.
It doesn’t really matter what value you set in there. The email router will recognize anything 0 or greater. If you put 0, it still sends.