If you are experiencing discrepancies between how your email looks in the Jeto preview (and even possibly the Marketo preview) and the way the final email looks in your inbox, it is most likely due to styling being carried over from the original source where the content was copied from.
Rich text fields allow for styling but can also capture some latent HTML styling from other sources if you copy/paste your text. Different email providers will have different ways of rendering this code and it can cause discrepancies in font style, colour or spacing. To avoid this, simply paste your content without formatting.
If you are using a MAC, you can do this by either hitting command + shift + v or by using the paste and match style option when right clicking on your mouse.
If you are using a PC, you can do this by hitting control + shift + paste or by using the paste without formatting option when right clicking on your mouse.