Why am I getting a mixed content warning when previewing an email?
If you aren't using the HTTPS Everywhere setting (Settings > Advanced > HTTPS Always), then images will be loaded over HTTP. This will cause a mixed content warning in most browsers:
This is generally something you can ignore because it only applies to the email being previewed within the MageMail system. When the email is actually delivered to customers and loaded in an email client, the email client handles images over HTTP just fine.