We know how frustrating this can be. Your signature looks great in a web browser (or in our 'Preview' pane), but there are issues with alignment, formatting, fonts, and/or image resolution after you create your signature. It's not our fault (we promise)! "But, it looks great in a web browser", you reply.
Unfortunately, HTML isn't a universal language. That means, HTML viewed in Safari may not match HTML viewed in Chrome, and viewing on PC, Mac, and Linux even with the same browser can yield different results (it's true)! This situation gets a lot more complicated when you consider there are over 50 email clients being used on web, desktop, and mobile and on a variety of different OS's (for some of the popularity rankings, see this list put together by Litmus).
This has been a thorn in the side of web developers for years, but web browsers are finally reaching a more level playing field. Unfortunately, email clients are at least a decade behind web browsers in terms of their HTML support. Although email standards have been proposed, they are often poorly implemented or ignored. Just because a signature looks great in your email client has no bearing on how it will appear within another email client. CSS blocks and custom fonts are just a few of the poorly supported features, and are not recommended for HTML for email.
Thus, developing HTML for email remains an art form and a skill, and is the reason that services such as Litmus, Email on Acid, and Preview My Email have been created. These sites actually send your HTML to each email client on a virtual machine instance and take a screenshot! This is the only way to know how the HTML will be rendered.
In summary, the 'Preview' functionality of the app is just to give you an idea of how your signature looks in Safari, but it must be viewed directly in the email client to see how it is actually rendered. There is no way to predict this other than to view it within the email client itself. The app just transfers your HTML "as is".
If you'd like personalized assistance with your HTML code, we offer paid support as well. Please contact us if you are interested in this option.