What is an SMTP Server?

An SMTP server accepts an outgoing message, finds the server responsible for the recipient's domain and hands the message over using the Simple Mail Transfer Protocol (RFC 5321). It queues mail it cannot deliver yet, retries on a schedule and sends a bounce if delivery finally fails.

The protocol is a plain-text dialogue. The client greets the server with EHLO, declares the envelope sender with MAIL FROM, each recipient with RCPT TO, then sends headers and body after DATA. Every server reply begins with a three-digit code: 2xx means accepted, 4xx means try again later, 5xx means a permanent refusal.

S: 220 mail.example.in ESMTP ready
C: EHLO shop.example.in
S: 250-mail.example.in
S: 250 STARTTLS
C: MAIL FROM:<orders@example.in>
S: 250 OK
C: RCPT TO:<buyer@gmail.com>
S: 250 Accepted
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>

SMTP only moves mail. It does not store an inbox or decide what you read; that is the job of IMAP or POP3, compared in the POP3 vs IMAP guide.

Use port 587 with STARTTLS or port 465 with implicit TLS in any mail client or website; leave port 25 to server-to-server relay. RFC 6409 defines 587 for message submission, and RFC 8314 recommends implicit TLS on 465.

A classic symptom: webmail sends fine but Outlook times out. The profile is still set to port 25, which the office or broadband network blocks. Change it to 587, STARTTLS and "my server requires authentication", and the same mailbox starts sending.

Route contact forms and order emails through an authenticated SMTP connection instead of PHP's mail() function. mail() hands the message to the local mail agent with no login, so the From domain often fails SPF alignment and the message lands in spam or vanishes.

In WordPress, an SMTP plugin replaces wp_mail()'s default transport. Enter the provider's outgoing host, port 587 or 465, the full mailbox address and its password, and set the From address to that same mailbox. Send a test to a Gmail address and open "Show original" to confirm SPF, DKIM and DMARC all read PASS.

Personal Gmail SMTP works for a low-volume form with an app password, but its daily cap and the gmail.com sender make it a poor fit for invoices. A mailbox on your own domain through WebJi® business email hosting sends with authenticated SMTP from an address that can pass SPF and DKIM.

The reply code tells you whether to fix credentials, DNS, content or volume, so read it before touching anything. A 4xx code is temporary and the server will retry; a 5xx code is final and needs a change on your side.

A connection that hangs with no code at all is almost always a blocked port rather than a wrong password. You can confirm from a terminal with openssl s_client -connect smtp.example.in:465 for implicit TLS, or openssl s_client -starttls smtp -connect smtp.example.in:587 for STARTTLS; a certificate and a 220 greeting mean the path is open.