Users can be blocked for going over usage limits (messages per day, recipients per message, etc)
https://support.google.com/a/answer/166852?hl=en
If you've been running a server for a long time, and you have no idea how it works, you might find that there are a LOT of messages in root's mailbox. Standard mailboxes are in /var/spool/mail/USER so <source lang="bash"> > /var/spool/mail/root</source> will truncate the mail file.
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