Cant see mail content after High Sierra

Hey there, KWT15.

It appears that you're having an issue with the Mail application on your MacBook Pro after updating to maxOS 10.13.3. When you try to preview an email you've received, it doesn't appear or it takes a while to show. I'm happy to help.

macOS Sierra: Quit apps -- Make certain the Mail app is completely closed, then restart the Mail app while holding down the shift key. If previews of your emails appear, quit the Mail application again, restart it normally and test.

How to turn on or turn off your Mac -- If the issue persists, restart your MacBook Pro then test the issue.

Use safe mode to isolate issues with your Mac -- If the previews still aren't appearing as expected, test how it responds in safe mode. If there are no issues, reboot normally and test again in your own user account. Remember this when booting in safe mode: Limited graphics performance in OS X recovery or safe mode

How to test an issue in another user account on your Mac -- If the issue is also happening in safe mode, use this article to create and test another user account. This will demonstrate if the issue is isolated to your account, or if it's system wide. You'll need to set up an email account for this new user.

Let me know how that goes.

Take care!

Let all mails get downloaded , it all depends upon the network speed .Once the process is completed click on a mail the subject must show .

Sometimes the mail account is deleted from internet accounts but that is to be done when mail app is malfunctioning , if in case you delete it and once again configure it see this article Set up Mail on your Mac with email accounts - Apple Support the same process of downloading will start .

So just wait and see what happens in the app .