If they aren’t receiving the transactions you've sent, we can create a test customer that has your email account. Then, send out a test invoice to yourself and see if you receive the email. If you've received it, let your customer check their spam folder.
If not, you'll need to reset your company email address. Here's how:
Go to the Gear icon.
Select Account and Settings under Your Company.
In the Contact info section, review each email address, then make changes as needed.