Do you have email delivery issues these days?

 Hello Raghad1, Welcome to the Community page, If you are receiving the undeliverable error to when trying to send the invoice to any of your customers then please refer to these articles-

 https://quickbooks.intuit.com/learn-support/en-uk/help-article/manage-customers/3-solutions-customers-receiving-emails/L9ypZGV4I_GB_en_GB

 https://quickbooks.intuit.com/learn-support/en-uk/help-article/email-campaign/receive-important-email-messages-intuit/L5edOwZ1P_GB_en_GB

Thanks