Apple MacOS and iOS users may be experiencing an error when viewing some Multipart HTML messages that states "This message cannot be displayed because of the way it is formatted. Ask the sender to send it again using a different format or email program." This is a known issue with Apple's implementation of IMAP (Internet Messaging Access Protocol) and although we do not know if there is a fix forthcoming from Apple the developers of our Mail Server software have released a workaround patch to remedy the issue for Apple IMAP users. We will be installing an out-of-band emergency patch to our Mail Servers on December 1st, 2017 between 12:00am and 2:00am during which all Mail Services may be experience a brief outage lasting approximately 5-15 minutes.
If you need to view an email that you are experiencing this error with prior to the patch installation on December 1st you can view the email from your web-based mail.
UPDATE 12/01/2017: The out-of-band maintenance window is complete as of 1:00am. Newly received Multipart HTML messages should no longer generate an error when viewing on Apple devices. For messages previously synchronized prior to the update if you move a problem message to another folder it will initiate a new synch with the Mail Server allowing you to finally view that Mutipart HTML message (after which you can safely move the email back to it's original folder if you desire).