Outlook (desktop versions) displays emails with Word's rendering engine, which does not support every layout: blocked images, frozen GIFs, rounded buttons turned rectangular, missing background images. No email marketing tool can guarantee identical display in Outlook, but the settings below, in the EmailBuilder of Sarbacane Campaigns, reduce the differences.
An email campaign created with the EmailBuilder of Sarbacane Campaigns.
An Outlook address and an address on another email service (Gmail, Yahoo…) for your tests.
At the Checks step, send a test to your Outlook address and to your other address.
Compare the two emails. If the email displays correctly elsewhere or in a browser, Outlook is the cause: find your symptom in Troubleshooting.
If you use two screens, close Outlook and reopen it on the main screen: the display can break on the secondary screen.
Best practice: keep a link to the online version in your email (Insert a link window, link type to the online version of the email). Your recipients can then open the campaign in their browser if it displays poorly in Outlook.
You know whether the difference comes from Outlook and which element to fix. After fixing it, send a new test to your Outlook address.
By default, Outlook blocks images hosted on a server. To display an image anyway, make it an embedded image: in Advanced settings, open the image hosting option, then choose embedded hosting for this image. Preferably embed only your logo: your recipients recognize you even if the other images are blocked, without making the email much heavier. Each embedded image increases the credit cost and can harm deliverability (see Why does my email campaign use more than one credit per recipient?).

Outlook limits image height. Reduce the image height, or split it into several parts, each in its own image block.
Outlook sometimes ignores resizing done in the EmailBuilder. Resize the image before importing it, with photo editing software or the built-in image editor.
Outlook can ignore margins around images. Add a border in the email's background color to the image with photo editing software, then import it again.

Outlook only displays the first frame of a GIF. Put the main information on this first frame.
Outlook does not support background images: a plain color replaces them. Do not place important text on a background image.
Google Fonts display poorly in most desktop email clients. Use system fonts (Arial, Helvetica…).
Outlook does not support rounded corners. To keep them, insert the button as an image. Since images are blocked by default, the button stays invisible until the recipient downloads them.
Some versions of Outlook resize the email according to the screen. Slightly change the height or width of the blocks concerned, then run new tests.
Blocks with 2, 3 or 4 columns shift depending on the screen resolution. At the Message step, in the EmailBuilder panel, open the Appearance section, then the settings section. Under the desktop version, reduce the width (650 px by default): misalignments usually disappear at around 400 px. Also limit the number of columns.
Outlook displays dotted or dashed borders on a white background. Use them on a white background, or switch to a solid border on a colored background.
Three habits make the email code heavier:
pasting text from another program (paste it as plain text instead, with Ctrl + Shift + V);
starting from the message of an old campaign (start from a template or saved blocks instead);
forwarding a received email.