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Email delivery is a sequence of handoffs: your mail app submits a message to an outgoing mail server, that server uses DNS to find a mail exchanger for the recipient’s domain, and SMTP transfers the message to that system. The receiving provider then decides whether to accept, filter, and store it. An SMTP success does not guarantee the message will land in the primary inbox.
How does email get delivered?
For an address such as person@example.com, the sender’s service routes mail using the domain after the @ sign—in this case, example.com. The mail app’s connection to its own outgoing service is a separate step from looking up the recipient’s mail server.
- The message is prepared and submitted. A mail application or service creates the message, including headers such as
From,To, andSubject, and sends it to an outgoing mail service. Submission commonly uses SMTP. - The sending system checks DNS. It queries for MX records belonging to the recipient’s domain to learn which mail exchanger should receive the message.
- The destination host is resolved. The sending system looks up an IP address for the chosen mail-exchanger hostname before connecting.
- SMTP transfers the message. The sending server exchanges SMTP commands and replies with the receiving server. One or more intermediate relays may handle the message on its way.
- The destination processes the message. The receiving system can accept it, defer it, or reject it. An accepted message may be stored in a mailbox or filtered into spam or another folder.
- The recipient accesses the stored message. A mail app or provider interface retrieves or displays it through a mailbox-access service. SMTP handles transport, not every part of the inbox experience.
The IETF describes SMTP’s objective as transferring mail “reliably and efficiently” in RFC 5321. That describes the protocol’s purpose, not a guarantee that every message will be accepted or shown in the inbox.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsWhat does an MX record do?
An MX (mail exchanger) record tells sending mail systems which host to try for incoming email addressed to a domain. A domain can publish multiple exchangers with preference values; the lower number is preferred. These values establish routing preference, not a priority for the recipient’s inbox.
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The selected MX hostname must resolve to an IP address so the sender can connect. If a domain has no MX record, RFC 5321 defines an implicit-MX fallback: the domain itself is treated as the mail host, subject to address resolution.
MX records route incoming mail. They do not authenticate the sender, guarantee delivery, choose a folder, or by themselves configure outbound sending. Exact record values vary by email host; use the records supplied by the service that will handle the domain’s mail. Cloudflare’s email-record setup guide likewise directs customers to obtain the relevant values from their email provider.
How does DNS route email?
DNS supplies the sender with the recipient domain’s MX hostnames and their preferences. The sender then resolves the chosen hostname to an address and opens an SMTP connection. DNS helps identify the next destination; it does not carry the message itself. SMTP performs the transfer.
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SMTP moves the message; message standards describe its contents
SMTP defines how mail systems exchange and relay messages. The message itself has headers and a body: RFC 5322 specifies the Internet message format, while MIME supports structured content such as multipart messages and attachments.
The visible From: header is not the same thing as the SMTP envelope sender identified with MAIL FROM. They can be related, but they serve different roles. This distinction matters when interpreting authentication results: SPF checks SMTP identities, while DMARC evaluates authentication in relation to the visible From domain.
What’s the difference between MX, SPF, DKIM, and DMARC?
| Mechanism | Main role | What it answers |
|---|---|---|
| MX | Routing | Which mail exchanger should receive mail for this domain? |
| SPF | Sender authorization | Is this sending host authorized for the domain identity used in SMTP HELO/EHLO or MAIL FROM? |
| DKIM | Cryptographic authentication | Does the message have a valid domain-associated signature, and has signed content remained intact? |
| DMARC | Alignment, policy, and reporting | Do SPF and/or DKIM authenticate in alignment with the visible From domain, and what policy or reporting instructions did the domain owner publish? |
SPF does not simply check the visible From address: RFC 7208 specifies checks against SMTP identities, notably HELO/EHLO and MAIL FROM. DKIM uses a cryptographic signature, with the public key published through DNS. DMARC connects SPF and DKIM results to the visible RFC 5322 From domain through alignment, and lets a domain owner publish policy and reporting instructions. See RFC 7489.
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These mechanisms have different jobs, and none guarantees primary-inbox placement. A receiving provider makes its own acceptance and filtering decisions.
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Why did an email go to spam instead of the inbox?
SMTP acceptance means a server accepted responsibility for a message at that point in the route; it does not prove that the final provider accepted it, stored it in the expected mailbox, or placed it in the primary inbox. A later relay can reject or defer the message, and a receiving system can filter an accepted message into spam or another folder.
When diagnosing placement or delivery, inspect the actual evidence rather than relying on the sender’s “sent” status:
Quick Recap
- Review any bounce or deferral notice and its SMTP response code.
- Check delivery logs to see which server accepted or rejected the message.
- Verify the recipient domain’s current MX answers and confirm the published values match the email provider’s instructions.
- Review SPF, DKIM, and DMARC results for the message, including whether authentication aligns with the visible From domain.
- Check the recipient’s spam or other folders; successful transport and authentication do not dictate folder placement.
What to check when configuring domain email
- Use the exact MX, SPF, DKIM, and DMARC records supplied for the email service you use; generic sample records are not safe substitutes.
- SPF is published as a DNS TXT record. RFC 7208 does not permit multiple SPF records at the same owner name; Cloudflare also identifies multiple SPF records as a configuration error for its service in its SPF, DKIM, and DMARC troubleshooting guide.
- DKIM selectors and public-key values are provider-specific. Copy the selector and key from the provider’s dashboard or documentation.
- DMARC is published as a TXT record at
_dmarc. Its policies can request monitoring, quarantine, or rejection. Validate legitimate sending sources before enforcing a restrictive policy. - After changing DNS, allow for caching and verify the answers from DNS rather than assuming every resolver has updated.
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