How to Fix Common Email Send Errors in Sparkforge

By Sparkforge Support Team ·Last reviewed May 3, 2026

Most failed emails come from unverified domains, missing MX records, or rate limits. Click the red triangle on any failed message to see the underlying SMTP error, then match it to the fix below.

Email delivery breaks for a small set of repeatable reasons: the sending domain is unverified, MX records do not resolve, the account is rate-limited, or the SMTP provider rejected the request. Click the red triangle icon on any failed message in Conversations to see the exact error, then jump to the matching fix.

Domain is not allowed to send

The sending domain is unverified and needs DNS configuration. In Mailgun, expand Sending, click Domain Settings, pick the right domain, open DNS records, and click Verify DNS settings. All five records should turn green. If they show green but mail still fails, click verify again to refresh.

Account Terminated

Mailgun has shut down the account. Contact Mailgun support directly to find the cause and reinstate.

Too old

Internal Mailgun error. Clear it from the Sparkforge agency Email Services tab, then contact Mailgun for the underlying reason.

Sender verification failed

Symptom: 550 Verification failed... Sender verify failed or Could not resolve sender domain. The recipient server cannot find MX records for the sending domain.

If you send from a subdomain

Point the subdomain MX records to Mailgun:

mxa.mailgun.org — Priority 10
mxb.mailgun.org — Priority 10

Wait 24–48 hours for propagation. If the error persists, set the From-field domain to match the sending domain.

If you send from a root domain

If the root domain already has MX records pointing to a mailbox provider, add a subdomain for Mailgun instead. Only ever point MX for one domain to a single source.

Last-resort fixes

Disable sender address verification on the recipient side.
Configure a catch-all on your email provider so verification passes.

Too many requests

You hit Mailgun's rate limit. Clear the error in Agency Settings → Email Services and contact Mailgun to confirm or raise your sending limit.

421 Temporary System Problem (Google Workspace)

Symptom: 421 4.3.0 Temporary System Problem. Try again later. Gmail SMTP throttles bulk sends. Either switch the user to Sparkforge Email or break the send into smaller batches.

Mailgun error reference (selected)

Code	Message	Fix
400	from / to / message parameter is missing	Populate the missing field on the message.
400	Too many recipients; max is 1000	Split the send into batches of 1000 or fewer.
400	Sandbox subdomains are for test only	Add and verify your own sending domain.
400	Domain is unverified — DNS configuration required	Verify all DNS records in Mailgun.
401	Forbidden / invalid API key	Re-issue and re-paste the Mailgun API key.
429	Too many requests	Throttle the send rate; raise plan limits if needed.

Other SMTP providers

Gmail, Outlook, and SendGrid each return their own error codes. The same diagnostic flow applies: read the error in the message detail, match it to the provider's published code list, and fix the underlying record, credential, or limit.

Frequently Asked Questions

Where do I see the actual error for a failed email?

Open the conversation, find the failed message, and click the red triangle icon. Sparkforge surfaces the raw SMTP error from the upstream provider so you can match it to a fix.

How long does Mailgun DNS propagation take?

Plan on 24–48 hours after you publish the records. Verification can succeed sooner, but sender verification errors often clear only after full propagation.

Why do my Gmail bulk sends return 421 Temporary System Problem?

Google Workspace throttles SMTP bulk volume. Switch to Sparkforge Email for bulk sends, or break the campaign into smaller batches with longer gaps.

Should I use a root domain or a subdomain for sending?

Use a subdomain like mail.yourdomain.com whenever the root domain already has MX records pointing to a mailbox provider. This avoids MX conflicts and keeps inbox delivery isolated from your sending reputation.

What does the Too old error mean?

It is an internal Mailgun state, not a delivery failure on your end. Clear it from Agency Settings → Email Services and contact Mailgun support for the root cause.