Konica Minolta SMTP 421 Error
SMTP 421 means the mail service is unavailable and closed the SMTP transfer channel for the bizhub C368/C308/C258.
Error meaning
The SMTP service is unavailable and closed the transfer channel.
This meaning is tied to the reviewed family/platform and subsystem below; do not reuse the number by itself on a different printer family or protocol.
Applies to
Konica Minolta bizhub C368/C308/C258 network services
- bizhub C368
- bizhub C308
- bizhub C258
Common formatting
These are formatting/lookup aliases for this reviewed condition, not separate error pages.
- SMTP 421
- 421
- SMTP421
What usually causes it
- The SMTP service may be temporarily unavailable or restarting.
- A gateway, relay, or security service can reject or defer connections during an outage.
- The configured server/port may be reachable at the network level but not accepting SMTP work.
What to try
Wait briefly and retry once
A 421 response can be temporary. Do not repeatedly resend a large batch while the server is unavailable.
Check whether the mail service is up
If your organization has a mail-status page or administrator, confirm the SMTP service is accepting connections.
Verify the configured SMTP host and port
Make sure the device still points to the intended server and service port.
Retry the original scan-to-email
After the service is available, resend the same small job before changing authentication settings.
After each corrective action
Retry the same operation that produced SMTP 421. If it now works, stop there rather than changing unrelated printer, network, or service settings.
If the code returns
If 421 occurs repeatedly while other clients can send through the same server, the mail administrator should review relay/service logs for the printer's connection.
When service may be needed
The code does not indicate an internal printer repair. Avoid changing certificates, credentials, or network security without evidence that those settings are wrong.
Continue with guided troubleshooting
If the printer still is not working after the reviewed checks, carry this verified error context into the guided troubleshooter.
Related information
Verified source
Official Konica Minolta bizhub C368/C308/C258 Error Code List; protocol context is part of the identity.