It is rare to find an email client that will support the display of iframe content, as it is a common vector for exploits on the internet.
Although we convert Google embeds to shortcodes for the purposes of displaying it on your site, your email client will interpret it as an iframe, and strip it from the message.
You can confirm this by viewing the RSS feed here and searching for "iframe"
If you are concerned about having a blank space in your emails, one possible workaround would be to adjust your feed settings (Appearance>Reading) and then be sure to add a Excerpt to your blog posts before publishing.
Your email will adopt this feed setting and show a bit of teaser text (the excerpt you added) as well as a link to read the full article, where they will be able to see the calendar as expected.
Hope that helps. Please let me know if you have any further questions.