You can do this under Settings > General > Company Profile.
Click on the relevant company profile and go to the 'Quote' or 'Invoice' tab:
User
If you choose 'User', the quote will always be sent from the person who sends the quote in Simplicate.
My company
If you choose this setting, the quote will be sent from the email address known on the CRM card of this company profile.
Other
If you choose 'Other', the quotes will always be sent from the email address you enter here, regardless of who sends it. In the case of the image above, the quote will therefore be sent from the email address: [email protected], with the name: Simplicate.
Please note: This function can only be used when the SPF records have been added to your own DNS.