Overview
Sometimes the formatting of an email you are trying to send is incorrect when building and testing it in Marketing Cloud. For example: the text scrolls off the screen while looking at the email in Outlook or another email client. This can happen whenever the formatting of objects in your emails, particularly images, are not set correctly
Detail
If the text or the image in an email scrolls out of the window of your email client Outlook, Gmail, Apple Mail) during testing in Marketing Cloud, it means formatting is off in the email.
An example appears below. Notice how the images aren’t lined up and at the bottom of the email there is a scroll bar.
Click image to enlarge:

It is best to review every image that you have in your email. It only takes one image with incorrect settings to throw off the entire email.
There are settings for dimensions for each image in the email. Since a standard email width is generally 600px, this should be the maximum width of any image. The image should also be imported into Marketing Cloud at the size it is intended to be used.
You can edit images in Email studio to be 600px width, but the safest option is to always format to correct size outside of Marketing Cloud. If a larger image is uploaded to Marketing Cloud (e.g. at 1200px wide), that’s the image that will display at that size in some recipients’ inboxes, This will force readers to scroll right to read the content, and the content will take twice as long to download.
Also, the option “Scale to Fit” is not supported in Outlook, so turning off this option is recommended.
Click image to enlarge:

If you adjust these settings, then your email will show up as it should.
Note - if the image is meant to be a half screen image- that would be 300 px.
Related Information
Salesforce provides additional advice on Best Practices and Tips on Marketing Cloud.
The Basic Email Formatting page on the Pitt Marketing Cloud Training Portal is also a valuable resource for training materials or knowledge: