When you receive this error especially upon previewing emails or opening emails, it usually hints at a broken image link.
How to fix
Move offending email to Junk email folder
- Move the particular email to junk before previewing it. Moving an email to the junk folder will automatically convert the email to text, removing the images
Force all emails to be viewed in plain text
- Another option similar to the above is to force all emails to be viewed in plain text. This might not be a good option if you like viewing emails that are formatted since it removes HTML as well.
Notify the sender that they have a broken link
- The last option would be to wait for the sender to fix this email by notifying them that their email has broken links. This error message is most likely popping up for anyone receiving email, especially if the broken link is an image part of their signature.