In order to override this default email client behavior, use the following HTML:
<a href="mailto:[email address]" style="[custom styling]">[email address]</a>
When the email address link is inside the anchor tag <a>, the email client will not override this custom styling that you wish to use.
This can be used for tokens as well:
<a href="mailto:{{lead.Email Address}}" style="[custom styling]">{{lead.Email Address}}</a>