Message cannot be displayed.... iPad

Message cannot be displayed.... iPad.

Periodically when I create an email on my iPad, I see this message when I access the email in my Sent folder or in my inbox when I have copied myself:

”This message cannot be displayed because of the way it is formatted. Ask the sender [that’s me!] to send it again using a different format or email program.

text/plain”

Recently this has occurred, not in any particular order though is happening more frequently. What might be the cause and how do I stop it?

Thank you.