With that said, you can code against some of these anomalies by adding extra code in CSS to account for the differences. Marketo Support is not set up to assist with troubleshooting custom code as we are not web developers. We would suggest that you consult with an HTML/CSS designer to make these changes.
To ensure that the existing email templates are coded to comply with different email clients, it is advisable to consult with a web developer/designer to review the code. Here is a guide that you may find helpful: https://www.campaignmonitor.com/css
Furthermore, even if the clients are the same, the act of forwarding an email can itself change the HTML of the email. However, Marketo does have a workaround for this. You would just need to put the ‘Forward to Friend' link directly into an email and when the person forwards the email using this link, there is a reduced possibility of the rendering issue occurring. It also adds the forwarded email address to your Lead Database as an unsubscribed lead.
The below article covers adding the ‘Forward to Friend’ link to your emails