What you’re seeing
- One user, or a shared mailbox such as billing@, suddenly gets every outgoing message returned with code 5.1.90.
- The bounce text says the mailbox has reached its daily limit for message recipients.
- Other employees can send normally, so the problem appears limited to that single account.
- The user insists they only sent a handful of emails, or it follows a mail merge or a reply-all to a large list.
Common causes
Every environment is different, but these are the causes our engineers find most often:
- Exchange Online caps each mailbox at a set number of recipients per rolling 24 hours, commonly 10,000, and every address on every message counts.
- A Word or Outlook mail merge sent individual copies to hundreds or thousands of contacts from one mailbox.
- Distribution lists expand to many recipients, so a few messages to large lists add up quickly.
- A connected app, scanner or script sends notifications through the mailbox far more often than anyone realizes.
- The account may be compromised and quietly sending spam, which uses up the allowance in hours.
Why it matters
A blocked mailbox is often the one that matters most: the owner, the sales lead or the accounts address that sends statements. Until the limit resets, that person cannot reply to customers or send quotes, and work gets rerouted through coworkers' accounts. If an attacker is behind the volume, the same account may be leaking data or phishing your clients, which turns a nuisance into a breach. Understanding the true cause keeps it from happening again next month.
How AskTechGurus fixes it
Skip the trial and error. A certified engineer works on your actual system, finds the root cause, and fixes it properly.
Tell us what’s happening
Submit a ticket in about two minutes. Paste the error, describe what changed, and pick your tier. No charge to submit.
A certified engineer diagnoses it
A vetted, US-based systems engineer claims your ticket, usually within minutes during business hours, and investigates over a secure remote session you can watch and end at any time.
Fixed and verified with you
The engineer resolves the root cause and confirms with you that it’s working. You don’t just get a workaround, and you get a plain-English summary of what was wrong.
Pay only when it’s resolved
Fixed price from $99, invoiced only after you approve the fix. If it isn’t resolved, you don’t pay.
Frequently asked questions
Can Microsoft raise the limit for us?
Recipient limits in Exchange Online are designed for person-to-person email and are not a channel for bulk mailing. Regular large sends belong on a proper email marketing platform.
How do I know if the account was hacked?
Unfamiliar messages in Sent Items, new inbox rules or sign-ins from strange locations are warning signs. In that case, assume compromise until an engineer confirms otherwise.
What would AskTechGurus check?
A certified US engineer reviews the mailbox's sending history and sign-in activity in a remote secure session. Pricing starts from $99 and you pay only when it's resolved.