SMTP (Simple Mail Transfer Protocol)

SMTP (Simple Mail Transfer Protocol) is the standard internet protocol for sending email messages between servers. When an application needs to send an email, it establishes a connection to an SMTP server, authenticates, and transmits the message. The SMTP server then routes the message through a series of mail transfer agents until it reaches the recipient's mail server. SMTP handles outbound delivery only — retrieving and reading messages is handled by separate protocols like IMAP and POP3. The protocol typically operates on port 587 (for authenticated submission) or port 465 (for implicit TLS).

For digital publishing platforms, reliable email delivery underpins critical workflows. Lead capture notifications, [OTP verification](/glossary/otp) codes, document approval requests, team invitations, and sharing confirmations all travel through SMTP. When emails fail to deliver or land in spam folders, publishers miss time-sensitive leads and readers lose access to gated content. Email deliverability depends not just on having an SMTP server, but on proper authentication — SPF, DKIM, and DMARC records that prove the sender is legitimate and authorized to send on behalf of the domain.

FlipLink uses SMTP-based email delivery for all outbound communications. When a reader submits their information through a [lead capture](/features/lead-capture) form, FlipLink sends the captured lead data to you via email. [OTP verification](/glossary/otp) codes, team collaboration notifications, document approval requests, and sharing confirmations are all delivered through SMTP. FlipLink manages the SMTP infrastructure internally, so you do not need to configure mail servers, set up authentication records, or monitor delivery queues. Emails are sent from verified sender addresses with proper SPF and DKIM authentication to maximize inbox placement.

If you are sending emails that reference your FlipLink publications (such as sharing links or marketing campaigns from your own email system), follow these steps to maximize deliverability: 1. **Configure SPF** — add a DNS TXT record for your sending domain that authorizes your mail server's IP addresses 2. **Set up DKIM** — generate a public/private key pair and publish the public key as a DNS TXT record so receiving servers can verify message integrity 3. **Enable DMARC** — publish a DMARC policy that tells receiving servers how to handle messages that fail SPF or DKIM checks 4. **Use port 587 with STARTTLS** — this is the recommended submission port for authenticated SMTP with encryption 5. **Monitor bounce rates** — high bounce rates damage your sender reputation and reduce future deliverability 6. **Keep mailing lists clean** — remove invalid addresses and honor unsubscribe requests promptly For emails sent by FlipLink itself (lead notifications, OTP codes, system alerts), FlipLink handles all of these steps automatically.

**"SMTP handles both sending and receiving email."** SMTP is exclusively for outbound delivery. Receiving mail is handled by IMAP (Internet Message Access Protocol) or POP3 (Post Office Protocol). When people say "email server," they typically mean a system running both SMTP for sending and IMAP/POP3 for receiving. **"If my emails reach the server, they will reach the inbox."** Reaching the recipient's mail server is only the first step. Modern email providers like Gmail and Outlook apply spam filtering, reputation scoring, and authentication checks before placing mail in the inbox. Without SPF, DKIM, and DMARC, even legitimate emails may be flagged as spam. **"SMTP is outdated and being replaced."** While SMTP dates back to 1982 (RFC 821), it remains the universal standard for email transmission. Modern extensions like STARTTLS (encryption), SMTP AUTH (authentication), and BIMI (brand indicators) have kept the protocol current. There is no replacement on the horizon.

An SMTP transaction follows a structured sequence: the client opens a TCP connection, sends a `HELO` or `EHLO` command to identify itself, authenticates with `AUTH`, specifies the sender with `MAIL FROM`, declares recipients with `RCPT TO`, transmits the message body with `DATA`, and closes with `QUIT`. Each step returns a status code (250 for success, 4xx for temporary failures, 5xx for permanent failures). Modern SMTP connections are encrypted using TLS, either via STARTTLS on port 587 or implicit TLS on port 465. Authentication mechanisms include PLAIN, LOGIN, and OAuth2, with OAuth2 becoming the preferred method for major providers like Google and Microsoft.