Read More Tag

If you’re using plain text then you can just insert the <!--more--> in your email.

If you’re using html then it is harder as the email clients add lots of markup.

You can use http://postieplugin.com/downloads/postie-shortcodes-addon/

Or you can roll your own using the hooks/filters. See https://postieplugin.com/extending/

Postie doesn’t have anything to do with your theme.

You might take a look at https://developer.wordpress.org/reference/functions/the_excerpt/