If Ignition is unable to deliver a proposal email to your client, you will receive an email that looks similar to this:
Email is usually fairly reliable, but there are a few common problems that can occur:
The email address of the contact the proposal is being sent to may have been misspelt
The email address of the contact may have changed
The email server that the recipient is using might be offline temporarily
Click the "View the proposal history" button in the email, and you'll be taken to the Audit Log for the appropriate proposal in Ignition.
If you think the problem is related to the contact email address, you can edit the client and update with the appropriate email.