Did you receive any emails that have image URLs embedded in them? If so, then your email app has to load those URLs in order to show you the images.
There's a setting in Mail's Preferences window that controls whether Mail.app's default setting. I think it's under the View or Viewing pane, and probably a checkbox that mentions "remote images" or something similar.
When the default state is Disabled, emails that have remote image URLs will be presented with a button called "Load Images" (or similar). You then manually click the button to load the images.
When the default state is Enabled, then emails with image URLs will automatically load the images. This obviously loads the images, but it also tells the server (or whoever runs the server) that you've looked at the message. It also tells them that their email has reached a live email account. In short, automatic image loading is a privacy leak, if one cares about such things.
There is a similar setting for Mail.app image URLs on iOS. It's in the Settings.app > Mail.
The above is just a guess based on your brief description. There may be other reasons for contacting those servers, but "loading image URLs" is my first guess.