I encountered an issue others may have which took me a minute to figure out.
When sending an email to all users, I kept getting an exception of an e-mail not in the correct format. I finally realized it was the Guest member which has a default e-mail address of "forum". To avoid this issue, you need to update this generic user to a valid email address format. This generic member should probably not be included when an admin selects "all users" in address field on the email page.
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