In this guide
Delivery and inbox placement are different
A receiving server accepting a message does not prove that the message reached the primary inbox, was read or was welcomed. Treat “sent,” “accepted,” “opened” and “replied” as different events. If a campaign has few replies, first check whether messages were accepted and whether the audience and offer make sense.
No domain age, warm-up schedule, word list or software setting guarantees inbox placement. An aged domain is not proof of a good sending history. Keep that distinction in mind when evaluating a mailbox service or a deliverability claim.
Check the sending identity
Ask whoever manages the sending domain to check SPF, DKIM and DMARC for the actual service sending your messages. SPF identifies authorized sending systems; DKIM signs messages; DMARC checks alignment with the visible From domain and publishes handling policy. A record existing in DNS is not the same as a real message passing authentication.
Inspect a test message's authentication results and compare the visible sender, sending service and Reply-To address. Follow the sending provider's configuration instructions rather than copying a DNS value from an unrelated account. Google's sender requirements cover authentication, transport and message-format expectations. Requirements differ by sender and traffic type, so consult the current guidance.
Check eligibility before attempting delivery
Document why an address belongs in the campaign. Keep opt-out requests, permanent delivery failures and administrator restrictions out of the sending audience. A format check or mail-routing DNS check is useful, but neither proves that an individual mailbox exists or that its owner wants the message.
When reviewing a list, distinguish addresses that failed a definite check from those with an inconclusive result. Do not label a timeout as proof that an address is invalid. Review unknown cases separately. Fixing a spelling mistake in one contact should not erase suppression history for other addresses.
Use a measured sending pattern
Begin with a small, eligible audience and inspect real results before increasing activity. Keep the sender identity stable and the reply mailbox monitored. If complaints or errors rise, pause and investigate; spreading the same problem across more mailboxes is not a solution.
Yahoo recommends permission-based sending and maintaining list quality in its best-practice guidance. Make unsubscribing straightforward and honor requests. Avoid repeatedly approaching someone who has already declined, regardless of which campaign or mailbox would send the next message.
Read errors before choosing a fix
Record the receiving server's response, the affected address or domain, and when the issue began. A temporary deferral and a permanent failure require different handling. Yahoo's SMTP error reference is an example of the provider-specific documentation to consult.
- Authentication failure: check the domain and sending-service configuration.
- Temporary deferral: follow the provider's instructions and your retry policy instead of repeatedly forcing retries.
- Permanent invalid-recipient failure: suppress the address and investigate the source of the list.
- Content or policy rejection: inspect the full response and message, including its links and attachments.
A short pre-send review
Confirm the audience, sender identity, authentication results, subject/body agreement, accurate signature, working links and unsubscribe route. Send a test to a mailbox you control. Preview the actual message rather than only the draft editor. Keep an example of the accepted message and any error response so later troubleshooting has evidence.
Once delivery is understood, use the engagement guide to evaluate replies and useful outcomes without confusing them with delivery statistics.
Frequently asked questions
Does mailbox warm-up guarantee inbox placement?
No. Warm-up cannot guarantee delivery, a good reputation or recipient interest. Review the sending identity, audience, content and actual provider responses.
Does a valid email format prove the mailbox exists?
No. Format and DNS checks cannot confirm every individual mailbox or establish permission to contact its owner.
What should I do when a receiving server rejects an email?
Read and retain the SMTP response. Identify whether it concerns authentication, the recipient, a temporary deferral or a policy issue before choosing a fix.
Should I retry a hard-bounced address?
Keep permanently failed recipient addresses suppressed. Do not repeatedly retry them or switch mailboxes to bypass the suppression.