E-mail: SMTP, IMAP, SPF, DKIM and DMARC

An e-mail's journey, from submission to the inbox, and why it lands in spam: SMTP, MX, IMAP, and authentication with SPF, DKIM and DMARC.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

What is SMTP?
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The protocol for sending and relaying e-mail between servers (port 25), and for submission by a mail client (587 with STARTTLS, or 465 over TLS).
It is only for sending: reading goes through IMAP or POP3.
Your billing e-mails land in spam at the big providers. First check?
- Send from a personal address
- That SPF, DKIM and DMARC are set up for the domain and pass
- Put the subject in capitals
- Change the SMTP port
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That SPF, DKIM and DMARC are set up for the domain and pass
The big providers now require sender authentication for bulk senders.
Setting DMARC straight to p=reject: a good idea?
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Risky: forgotten legitimate senders (newsletter, third-party tool) would be rejected. Start with p=none and read the reports.
Aggregate DMARC reports list the servers sending for the domain, with their SPF and DKIM results.
Sending an e-mail: after “Authenticated SMTP submission on port 587”, what is the next message?
- MX answer: the domain's receiving server
- SMTP: EHLO, MAIL FROM, RCPT TO, DATA with a DKIM signature
- Lookup of SPF, the DKIM key and the DMARC policy
- MX query for the recipient's domain
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MX query for the recipient's domain
The server looks up where to deliver the message.