In the email development industry, there are two methods of building mobile-friendly emails:
by using CSS Media Queries, let's call it “Responsive”;
without Media Queries, let's call it "Fluid-hybrid."
The actual problem is that some mobile email applications still don’t support responsive design.
We are aware of this problem and even published a dedicated blog post.
So what happens during the export process? You click the Export button, email is sent right to the desired email client -- and if the responsive design (Media Queries) is not supported, all the media styles are automatically removed making the email non-responsive for mobile apps.
As we know Gmail removes all CSS-code from responsive styles and that's why you see a web-version of your email on mobile devices.
To ensure correct display of emails in Gmail, We recommend that you export your email to any ESP which supports media-queries and then send it to Gmail. In this case, the email would be responsive in this email client.
The situation with Outlook is quite similar -- not all versions of Outlook support responsive design (mobile-friendly design) -- it just automatically removes all responsive styles from the code.
That is why some email clients show fully responsive (mobile-friendly) emails on mobile devices and Outlook app on mobiles may show the Web-version of the same email.
Please be advised that all the test emails you send from Stripo to Outlook.com will be responsive on mobile devices. However, in Outlook Apps the same email may be non-responsive.
If you have any questions, please contact us via email at email@example.com.