454: too many connections (rate control)

What it means

Barracuda gateways limit how fast one sending source may connect, a feature their documentation calls rate control. This reply means your server went over that limit. Barracuda also lists a second form, too many connections to server, for the same feature. It is temporary: your server keeps the mail and tries again, and delivery resumes once the rate drops. A sender that keeps hitting the limit spends most of its time being deferred, so lowering concurrency to that company is faster overall than pushing harder.

Common causes

• Many messages to one company sent at once over parallel connections.
• Several senders sharing one outgoing IP.
• A retry storm after an earlier failure.

How to fix it

1. 01Let the queue retry; the deferral lifts once your rate falls.
2. 02Lower the number of simultaneous connections to that domain in your mail server or sending tool.
3. 03Spread large sends to one company over a longer period.
4. 04If you need a higher rate, for example as a known partner, ask the recipient’s IT team about an exemption.

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

Further reading

• Soft bounce vs hard bounce
• The standard meaning of 454: temporary authentication or TLS failure