The layer cake (specific wins over general)
Org config → mailbox/CAS PLAN (per licence tier) → per-mailbox overrides. Plans are the under-documented middle: templates applied at licensing time (four in a typical tenant — our scan lists them, and our POP/TLS verdict mapping reads them). Change a plan and you change FUTURE mailboxes; existing ones keep their stamped values — the classic 'new starters behave differently' explanation.
The numbers that actually bite
| Limit | Typical value | Symptom when hit |
|---|---|---|
| Mailbox quota (by licence) | 50GB / 100GB tiers | Prohibit-send at threshold; 5.2.2 inbound bounces beyond |
| Message size | 25MB default, raisable to 150MB (client paths vary) | 'Attachment too large', both directions — raise deliberately |
| Recipient rate | 10,000 recipients/day | The app or marketing mailbox that dies mid-campaign — HVE/ACS territory (auth module) |
| Message rate | 30 messages/minute submission | Scripted senders throttle-crawling |
| Recipients per message | 500-1,000 by config | One send, hundreds of NDRs |
| Deleted-item retention | 14 days default → 30 max | Recovery windows shorter than users assume |
The philosophical point: these are ANTI-ABUSE rails, not capacity dials — the fix for hitting rate limits is almost never 'raise it' (mostly you can't) but 'use the right pipe': HVE internal, ACS external, Graph for apps (protocol-retirements concept).
What to watch (proofs)
- The plan truth: Get-MailboxPlan | fl Quota and Get-CASMailboxPlan (both in the nightly scan — plan drift is a dated diff, which mattered the day our TLS mapping fired on PopEnabled).
- Effective values per mailbox: Get-Mailbox | fl Quota, MaxSend*, MaxReceive*, RecipientLimits — stamped reality vs plan template; deltas are deliberate overrides or archaeology.
- Quota pressure fleet-wide: the near-quota report — next month's 5.2.2 bounces, visible this month (Pulse's capacity row).
- Throttle encounters: submission logs + trace DEFER patterns at metronomic rates — the signature of a limit, not an outage.