@reply and @email keywords
SendBoard has a feature that allows you to send an email by adding a Trello comment. You do this by starting the comment with either the @reply or @email keywords, depending on if you are replying or starting a new email. There are other special rules to set email recipients, set the email subject and include Saved Replies, Variables and Custom Fields.
Read more about these keywords: Send an email by adding a Trello comment
Use Butler for email automation in Trello
Butler is a powerful utility for automation in Trello. One of the things that Butler can do is automatically add a comment to a card based on triggers or rules.
So if you use the @reply/@email keywords in a Butler rule, you can set up automations to send emails when certain events occur.
Example - Send an email when you drag a card
Here is an example Butler rule that will send out the Saved Reply with keyword "shipped" to the contact on the card as soon as the card is dragged into the "Done" list. The value of the "Order Number" custom field will be inserted into the subject of the email.