Emails displaying differently in certain email clients is an issue that all ESPs experience, most commonly with Outlook. The way an email displays is dependent on the way the email client renders that email’s HTML. Even if you build your email in a drag-and-drop editor, your ESP converts it into HTML before it’s sent out. Each client then parses the HTML and decides what to render. They all have their own quirks: HTML elements aren’t recognized or styles are rendered differently. This is especially prevalent in old versions of email clients that are downloaded to a device (vs. accessed in a browser, and therefore kept up to date).
Some clients, like Outlook, have hundreds of past versions that could be running on a recipient’s device, and optimizing for any one of them requires significant conditional HTML additions. All ESPs therefore have to prioritize certain clients over others in the way that they optimize a template’s HTML. Both Mailchimp and Klayio have addressed the issue of how emails display in Outlook as well.
You can use an email template testing service like Litmus to see how your template will display across all email clients. Try sending yourself a test email from your ESP using a Litmus test email address and then optimize for the email clients that are most commonly used by your subscribers.