The status of an user account is displayed with an icon next to the account's associated email address, for example in the user profile view or user list. The user account status:
is automatically set based on the status of the email address (unverified, temporary unusable or inexistent)
can be also set by the school admin, for example by "freezing" an account
The user account status icons and their meaning
a skull icon - the email address is invalid, such mailbox does not exist
a yellow triangle - the email address may exist but messages cannot be delivered by the intended recipient, typically a temporary problem as as mailbox full, for example.