In Microsoft Outlook you get the error:-
The attached message had PERMANENT fatal delivery errors!
After one or more unsuccessful delivery attempts the attached message has been removed from the mail queue on this server. The number and frequency of delivery attempts are determined by local configuration parameters.
YOUR MESSAGE WAS NOT DELIVERED TO ONE OR MORE RECIPIENTS!
Near the bottom of the huge error message it states
Outlook 500 Line limit exceeded
1. Open the email you tried to send
4. Change the option next to “Internet Format” to “Send plain Text only”
This error message occurs to email addresses on 1&1 Hosted Mail Servers and this is the only resolution to the problem