From the Jobs / Orders List, select the checkboxes next to the orders that you would like to send emails to.  Then, click the 'Email' button.  (Limited to the amount of records displayed on your Jobs / Orders List, maximum 100).

Once you've selected your email and invoice templates to send, click 'Send'!
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NOTE: Bulk email is not available on the Estimates list page. Bulk email for estimates is not possible due to the multiple sub-proposals feature.