If emails are not going out; stuck in the queue

If you find that your emails are stuck in the queue; at least with Postfix, make sure you have set up an email address that matches the one in Dolphin's admin for the from address for site emails.  That user has to exist on the system.

you mean along the admin@mysite.com people should also create no-reply@mysite.com to send emails.

More of, don't set your site's notification email to an address that doesn't exist on the server. So for example.com:

* myemail@gmail.com: Bad

* info@example.com: Good

Of course, the email address needs to be created first.

What I stated; if running a Postfix mail server and in your admin you have it set for some-name@some-site.com to be the return email address listed in the site emails to your member, then that address/user should be present on your system/server or the emails will not be processed.  The error message in the logs would have one thinking it was a DNS error when it is simply the user does not exist on the server.