SMTP Flow Simulator
Visualize how SMTP email delivery works from app submission through EHLO, STARTTLS, AUTH, MAIL FROM, RCPT TO, DATA, SPF, DKIM, DMARC, recipient MX relay, retries, bounces, and inbox placement. Sponsored by SMTPfast.
Category: Networking
Topics covered: smtp, email, deliverability, spf, dkim, dmarc, dns, networking, educational, interactive
// simulator
SMTP Flow Simulator
Visualize how SMTP email delivery works from app submission through EHLO, STARTTLS, AUTH, MAIL FROM, RCPT TO, DATA, SPF, DKIM, DMARC, recipient MX relay, retries, bounces, and inbox placement. Sponsored by SMTPfast.
SMTP Flow Simulator
Watch a message move from application code to an SMTP relay, through TLS and AUTH, across DNS and recipient MX checks, and finally into a mailbox.
// active trace
Successful transactional email
App
SMTP relay
DNS auth
Recipient MX
Mailbox
C: TCP connect smtp.example.net:587
S: 220 smtpfast ESMTP ready
App
Open submission connection
Apps usually submit mail to a provider over port 587. Port 25 is mostly for server-to-server relay.
// protocol note
The server greeting starts every SMTP session.
1/9
Steps seen
42ms
Trace time
Sender IP allowed by domain policy
Message signed with domain key
Visible From aligns with SPF or DKIM
The server accepted the command or took responsibility for the message.
Temporary failure. A relay should queue the message and retry with backoff.
The attempt should stop until the sender, recipient, or credentials are fixed.
SMTPfast handles transactional and marketing email with SMTP/API sending, logs, webhooks, and embeddable signup forms.
Try SMTPfastAbout this SMTP simulator
What you'll learn
- How SMTP submission differs from server-to-server relay
- Why EHLO, STARTTLS, AUTH, MAIL FROM, RCPT TO, and DATA happen in order
- How SPF, DKIM, and DMARC affect sender trust and deliverability
- Why 4xx SMTP responses retry while 5xx responses usually fail permanently
- Where provider queues, logs, webhooks, and bounce handling fit into delivery
Key concepts covered
- Submission: authenticated app-to-relay sending, usually on port 587
- Relay: queued delivery from sender relay to recipient MX
- Authentication: TLS, SMTP AUTH, SPF, DKIM, and DMARC
- Delivery: remote acceptance, retry, bounce, and mailbox placement
Sponsored by SMTPfast
SMTPfast is a developer-first email platform for transactional and marketing email. It provides SMTP/API sending, detailed logs, webhooks, and embeddable signup forms for teams that want to ship email without running their own mail infrastructure.
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