Outlook on the Web (what used to be called OWA) has a useful feature that checks an email you’re about to send, and if it contains words that indicate that there should be a file attached to the message, will warn you that nothing has been attached. It’s very nice, and it’s something we’ve all done once or twice or a few hundred times over the years, so I certainly appreciate the warning.
Included in the warning is that tick box to tell Exchange “Don’t show me this message again.” Obviously that tick box exists to cater to what is surely a small number of people who frequently mention words like “attachment” or “attached” without actually attaching anything to the email message, and therefore don’t want to be nagged about it all the time. But of course, it also means that normal people are going to tick that box by accident, and then complain when “that warning that usually appears didn’t show up and I accidentally sent an email without an attachment!”
Fortunately, the setting is controlled in the message configuration for the mailbox as an attribute named CheckForForgottenAttachments, which can be checked by running Get-MailboxMessageConfiguration.
[PS] C:\>Get-MailboxMessageConfiguration -Identity email@example.com | Format-List CheckForForgottenAttachments CheckForForgottenAttachments : False
Set the attribute back to True to re-enable the forgotten attachment check.
[PS] C:\>Set-MailboxMessageConfiguration -Identity firstname.lastname@example.org -CheckForForgottenAttachments $true
The user may need to log out and in to Outlook on the Web, or at least do a refresh in their browser, for the setting to take effect again.