Q: cannot find mail connection doctor

I am having a problem sending email msgs and the online world has many posts saying that this is likely due to an issue with the El Capitan upgrade. In order to fix the problem, I need to use Mail Connection Doctor utility, but I can't find it anywhere on my computer. I have Mac OS X - El Capitan version 10.11.6. (I have a 2008 iMac computer. It's a single user NON networked desktop computer and I'm the only user. It has automatic updates selected.)  Is it possible that the MCD wasn't downloaded when the current version of the OS was added to my computer? Is there anywhere I can go to add it to my computer? HOW do I access the MCD utility??? Help please