Problem
You don't know which estimate or invoice an email corresponds with.
Solution
Add the {InvoiceNumber} and {EstimateNumber} placeholders to your Emails
Placeholders let you add customer-specific content to your templates. If you want to know which invoice or estimate you're referring to when sending multiple emails, add the {InvoiceNumber} and {EstimateNumber} placeholders to the Subject fields of your Invoice and Estimate email templates.
Go to Settings
.Go to Invoicing > Email.
On the Email settings screen that opens, select the email template you wish to edit.
To include the invoice number in the subject line of your invoice emails, copy and paste the {InvoiceNumber} placeholder into the subject line. You can also include the {InvoiceNumber} in the body of the email.
To include the estimate number in the subject line of your invoice emails, copy and paste the {EstimateNumber} placeholder into the subject line. You can also include the {EstimateNumber} in the body of the email.
This way, when a customer responds, you'll immediately know which invoice the reply pertains to.