SMTP

SMTP (Simple Mail Transfer Protocol) is the internet standard for transferring electronic mail between servers, originally defined in RFC 5321 (2008, superseding RFC 821). It is a text-based, client-server protocol: a sending server (the SMTP client) opens a TCP connection to a receiving server (the SMTP server) and exchanges a fixed sequence of commands — HELO/EHLO, MAIL FROM, RCPT TO, DATA, QUIT — to transmit a single message.

Three TCP ports are commonly used:

• Port 25 — server-to-server SMTP relay. Most consumer ISPs block outbound traffic on port 25, so end users do not normally use it.
• Port 587 (Submission) — authenticated message submission (RFC 6409), upgraded to TLS via STARTTLS.
• Port 465 (SMTPS) — implicit TLS from the connection start, formally re-blessed in RFC 8314.

Modern operators authenticate on 587 or 465; both are typically referred to as "SMTP credentials" by ESPs.

AcelleMail abstracts the wire protocol behind a polymorphic SendingDriver contract (app/SendingServers/Drivers/SendingDriver.php). Built-in drivers include a generic SMTP driver that speaks RFC 5321 against any compliant server, plus vendor-specific SMTP drivers for Amazon SES (AmazonSmtpDriver.php), Mailgun, SendGrid, SparkPost, and others. The vendor drivers wrap the same SMTP exchange but pre-fill the credentials and endpoint, so an operator chooses "Amazon SES SMTP" instead of typing email-smtp.us-east-1.amazonaws.com manually.

For new vendors that ship only a REST API (no SMTP endpoint), AcelleMail also supports an API driver path — same SendingDriver contract, different transport.

Practically every server today speaks ESMTP (Extended SMTP, RFC 5321 §2.2) — the EHLO greeting advertises optional extensions like SIZE (max message bytes), 8BITMIME (8-bit clean transport), STARTTLS (opportunistic TLS upgrade), and AUTH (SASL authentication). The colloquial term "SMTP" almost always means ESMTP in a modern conversation.