I've been having the same "Delivery Server Down" issue all morning too. I tried removing my attachments, reducing the "TO" email address to just my own as a test, etc. Nothing had changed about the way I send my invoices to this customer, which has been every single month for the last 2 years. I never had an issue with this one prior. And I know it's not hitting them, because when it does send successfully, I get an automated message from their end from Jira acknowledging.