Hello,
So I have been dealing with this problem for almost a month now and have done a lot of research. I am having issues sending workflow emails or sending emails from CRM. They are getting stuck in Pending Send status. All the users in my organization are having this issue.
Through my research, I have checked the CRM Diagnostics settings, which seem to be correct:

The Email Server Profile has been set up for server side synchronization :

I have made sure that everyone in the organization in their Outlook Client for CRM has checked off the option to allow CRM users to send email on your behalf:

Now, in the CRM Email Configuration Settings, this is where it gets complicated. Even though we are using Server Side Synchronization, we have set that up for incoming and outgoing Email. However, the Process Email Using is set to MS Dynamics CRM 2015 Email Router.

Now when we switch this to Server-Side Synchronization, all of the pending emails go through, and emails are able to be sent through CRM. However, from the Outlook Client, we are no longer able to track emails or set the regarding from Outlook. We need to do both, not one or the other.
I really need help trying to figure this out, because we are tracking emails from Outlook, converting them to cases, and/or setting the regarding to link them to opportunities, but we have several workflows that send out emails that need to be working as well.
Any thoughts?
Thanks!