Change the e-mail message if required.
'To' address is the e-mail address of the contact person you have specified for the customer. If no contact e-mail address was provided, this field will be left blank and you need to specify an e-mail address.
You can attach an associated document like sales order, a project document, etc. to an estimate. To do this, click on the 'Attach a file' link below the e-mail message box and browse your local folders to select the document that you want to attach. The document size should not exceed 10 MB.
Zoho Invoice FAQ