Select the proposal you wish to email and click Manage.
Click the Email button in the upper-right and a new window will appear where you can set the recipient, subject line, and message (these are all filled in by default but you can edit any you choose).
If you check the Send Copy to Business Email box, a copy of the proposal will also be sent to your business email (set under Settings > Store Info) for your records. You may opt out of the copy by unchecking the box.
Once you're happy with the form, hit Send to send the email.