A mailbox has three thresholds, and the gaps between them explain the symptoms.
| Threshold | Default (E3/E5) | Effect |
|---|---|---|
| Issue warning | 98 GB | Warning message to the user; everything still works |
| Prohibit send | 99.5 GB | Cannot send. Can still receive |
| Prohibit send/receive | 100 GB | Cannot send or receive; senders get an NDR |
"I can receive but everything sticks in my Outbox"
That asymmetry is the ProhibitSend threshold, and it is diagnostic on its own. The
client is fine and connected; the service is refusing the submission:
550 5.2.2 Submission quota exceededNothing has been lost. The messages sit in the Outbox and go as soon as the mailbox
drops back under the threshold.
The primary mailbox limit is fixed — 100 GB on E3 and E5. No licence raises it.
The only route is to move content out.
Immediate relief
- Empty Deleted Items and Junk Email, which are counted.
- Empty Recoverable Items if it is bloated — beware, this is not possible under
litigation hold, and it is a different remedy with compliance implications.
- Move the largest recent items to the archive once it exists.
The durable fix: an online archive
The archive is a second mailbox that appears in Outlook as a separate folder tree,
stays fully searchable, and does not count against the primary quota.
- E3 and E5 include Exchange Online Archiving with auto-expanding storage. It is
frequently just never switched on for the mailbox.
- Apply a retention policy with an archive tag — for example, move anything older
than two years — so it happens automatically from now on.
- The Managed Folder Assistant works through the backlog in the background. On a
large mailbox this takes days, not minutes.
Say clearly to the user: nothing is deleted. People with fourteen years of
project mail hear "archive" as "delete" and dig in.
Do not
- Do not rebuild the Outlook profile. The rejection is server-side; a rebuild
costs the user hours of re-sync and leaves the quota where it was.
- Do not tell the user to "delete some emails" and close the ticket. That is the
same conversation every eighteen months.
- Do not add a licence expecting a bigger primary mailbox. The 100 GB limit does
not move.
Related
- Troubleshooting an Outlook profile
- Running a message trace