The feature is available since Plesk Obsidian Release Candidate 3 (RC3 18.0.17) https://docs.plesk.com/release-notes/obsidian/change-log/#contents-18-rc30
Please check and provide your feedback.
And how do you want to email them if their mailbox size is exceeded?
You want exceptions? Exceptions can be exploited, it'll result in a full hard disk and far more issues than a full mailbox.
It's better to inform your customer in a descent way instead of just shoving accounts for the lowest price and no attention to mailbox ethics.