5.1.90: daily recipient limit reached

What it means

Exchange Online caps how many recipients one mailbox can send to in 24 hours, to discourage unsolicited bulk mail. When the cap is reached, further messages bounce with 5.1.90 until earlier sends fall outside the window. Microsoft warns that reaching the limit can mean the account was compromised and is being used to send spam. For legitimate bulk commercial email, it recommends third-party providers that specialize in it, or Azure Communication Services for high-volume mail to external recipients.

Common causes

• The user sent to more recipients in 24 hours than the recipient rate limit allows.
• The account is compromised and sending spam.
• An application uses a user mailbox for bulk sending.

How to fix it

1. 01Check Sent Items for messages you did not send; if you find any, report a possible compromise to your admin.
2. 02Wait for the 24-hour window to move on before sending more.
3. 03Move newsletters and bulk sends to a service built for them.
4. 04Admins: if the volume was not legitimate, follow Microsoft's steps for responding to a compromised account.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
• Exchange Online limitslearn.microsoft.com