Annoying "This message couldn’t be sent because you haven’t specified any recipients" has started to appear

I have just started to get the following message whenever I send emails. Yes, the emails are addressed and s in fact do get sent but this is incredibly irritating. Nothing has changed with my computer or settings or email.

"This message couldn’t be sent because the following internal error occurred: This message couldn’t be sent because you haven’t specified any recipients".

I am not a technical person so any advice would need to be in simple language!

Cheers