421 4.3.240: HVE: too many simultaneous connections

What it means

High Volume Email (HVE) in Exchange Online lets applications and devices send large volumes of internal mail through dedicated HVE accounts. Microsoft allows up to 250 simultaneous authenticated connections, a limit its HVE page applies per tenant, and up to 100 per IP address. Opening more than 250 triggers 421 4.3.240 and the service closes the transmission channel. It is a temporary error: the client can reconnect and send once it keeps its connection count under the limit.

Common causes

• The sending application opens a new connection for each message instead of reusing connections.
• Several applications or instances in the tenant together exceed 250 connections.

How to fix it

1. 01Keep parallel SMTP connections to the HVE endpoint below 250 for the tenant and 100 per IP address.
2. 02Reuse each connection to send several messages.
3. 03Let the client retry the refused messages after the connection count drops.

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
• Manage High Volume Email for Microsoft 365learn.microsoft.com