421: too many messages in one session
Your server sent more messages over a single connection than the Barracuda gateway allows, so it closed the session. The remaining messages retry on new connections.
- Barracuda
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 421
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
421 Error: too many messages in one session
What it means
Barracuda gateways cap the number of messages a sender may deliver in one SMTP session. Once your server reaches the cap, the gateway answers with this reply and the session ends. It is temporary: messages not yet delivered stay in your queue and go out on later connections. On its own this costs a little time. Combined with a strict connection limit, it can slow delivery to that company a lot, so it is worth matching your server’s messages-per-connection setting to the receiver’s limit.
Common causes
- Your mail server sends many messages over one connection to the same domain.
- A low per-session limit configured on the recipient’s gateway.
How to fix it
- 01Let the remaining messages retry on new connections.
- 02Lower the maximum messages per connection for that destination in your mail server.
- 03If delays build up, slow your sending rate to that company.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- SMTP Error Codes, Barracuda Email Security Gateway documentationdocumentation.campus.barracuda.com
Related codes
- 421Service not available.The receiving server is closing the connection and will not take mail right now. The reply is temporary, so the sending server keeps the message and tries again later.
- 454Too many connections (rate control).Your server opened more connections to the Barracuda gateway than its rate control allows, so the gateway is turning it away for now.
421 at other providers
- 421 [RP-001]Outlook.com rate limit for the sending IP.Outlook.com is limiting how fast your IP can send to it because of the IP or domain's reputation; it is temporary and your server should retry.
- 421 [RP-002]Outlook.com rate limit on this connection.Your IP has sent more over a single connection than Outlook.com allows, a limit Microsoft ties to IP and domain reputation.
- 421 [RP-003]Outlook.com connection limit.Your IP has opened more connections to Outlook.com than it is allowed, a limit Microsoft links to IP and domain reputation.
- 421Temporary error or deferral.Yahoo or AOL refused your message for now with a 421, one of its two temporary error codes. Your server should keep it and retry; repeated deferrals point to reputation or complaints.
- 421Try again later.Xfinity turned the message away for now with a generic 421. Treat it as temporary and let your server retry: Xfinity uses it for several reasons it does not spell out.
- 421Reverse DNS lookup failed.Xfinity could not complete a reverse DNS lookup for your sending IP. It treats this as temporary, so your server should retry, but a broken DNS setup keeps it failing.
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