Swaks: Test WordPress SMTP from the Command Line

WordPress email problems have three moving parts: the plugin that triggers the email, the mailer plugin that routes it, and the SMTP server that sends it. When mail breaks, the fastest way to find the fault is to remove the first two and talk to the SMTP server directly.

Swaks (Swiss Army Knife for SMTP) does exactly that. It is a single-file Perl script that opens an SMTP session from the command line, testing connectivity, authentication, TLS, and delivery without WordPress or PHP in the loop.

GitHub — jetmore/swaks

Assumptions

This walkthrough assumes:

1. The mail server accepts messages over SMTP, not a provider-specific API (Mailgun HTTP API, Amazon SES API, etc.). API-based connections bypass SMTP entirely and cannot be tested with Swaks.
2. You have SSH access to the server where WordPress runs and are comfortable with the Linux command line.

What is Swaks?

Swaks is a command-line SMTP test tool written in Perl. It connects to an SMTP server, runs through the SMTP transaction (EHLO, AUTH, MAIL FROM, RCPT TO, DATA), and prints every line of the exchange. That transcript is the diagnostic: it shows exactly where the conversation succeeds or fails.

Relevant capabilities for WordPress SMTP testing:

• Authentication methods: LOGIN, PLAIN, CRAM-MD5, DIGEST-MD5 (covers every method WordPress mailer plugins use).
• STARTTLS and implicit TLS.
• Custom headers, body, and attachments.
• Full SMTP transaction log with server response codes.

Prerequisites

1. SMTP credentials: hostname, port, encryption type, username, and password for the mail server your WordPress site sends through. These are the same values configured in your mailer plugin (WP Mail SMTP, FluentSMTP, Post SMTP, etc.).
2. Swaks installed on the WordPress server: install from the official instructions or your distribution’s package manager (apt install swaks, yum install swaks). Install on the same server as WordPress — this ensures the test reflects the same network path, firewall rules, and DNS resolution that WordPress uses.

Testing WordPress SMTP with Swaks

The full option reference is in the Swaks documentation. The examples below build up incrementally — connection first, then authentication, then a full send — so each step isolates one variable.

A complete Swaks command looks like this:

swaks --to [email protected] --from [email protected] --server smtp.example.com --port 587 --auth LOGIN --auth-user "username" --auth-password "password" --tls

Storing this in a shell script (swaks_test.sh) avoids retyping credentials on every run.

Step 1: Test the connection

Create swaks_test.sh with a bare connection test. Replace smtp.example.com and 587 with your SMTP server’s hostname and port:

#!/bin/bash

swaks \
--server smtp.example.com \
--port 587

Save the file and make it executable:

chmod +x swaks_test.sh

Run the script from the command line:

./swaks_test.sh

If the connection succeeds, Swaks prints a To: prompt — it connected and is waiting for a recipient. Press Ctrl+C to exit.

If it fails here, the problem is network-level: wrong hostname, wrong port, firewall blocking the connection, or the hosting provider blocking outbound SMTP (common on shared hosts — see the troubleshooting section below).

Step 2: Test email delivery

Add sender/recipient addresses to test whether the server accepts a message. Replace the placeholder addresses with real values:

#!/bin/bash

swaks \
--to [email protected] \
--from [email protected] \
--server smtp.example.com \
--port 587

The server will reject the message with 530 5.7.1 Authentication required — this is expected. The point of this step is to confirm that the SMTP session reaches the MAIL FROM stage, proving that the connection and EHLO handshake work:

== Trying smtp.example.com:587...
=== Connected to smtp.example.com.
<-  220 smtp.example.com ESMTP ready
 -> EHLO yourserver.hosting.com
<-  250-smtp.example.com
<-  250-SIZE 5242880
<-  250-PIPELINING
<-  250-ENHANCEDSTATUSCODES
<-  250-8BITMIME
<-  250-DSN
<-  250-AUTH PLAIN LOGIN CRAM-MD5
<-  250 STARTTLS
 -> MAIL FROM:<[email protected]>
<** 530 5.7.1 Authentication required
 -> QUIT
<-  221 2.0.0 Bye
=== Connection closed with remote host.

Note the 250-AUTH PLAIN LOGIN CRAM-MD5 line in the EHLO response — it lists the authentication methods the server supports. Match the --auth flag in the next step to one of these.

Step 3: Test authentication and TLS

Add authentication and TLS. Use the same credentials configured in your WordPress mailer plugin:

#!/bin/bash

swaks \
--to [email protected] \
--from [email protected] \
--server smtp.example.com \
--port 587 \
--auth LOGIN \
--auth-user "username" \
--auth-password "password" \
--tls

A successful authentication and delivery produces 250 2.0.0 Ok: queued:

=== Trying smtp.example.com:587...
=== Connected to smtp.example.com.
<-  220 smtp.example.com ESMTP ready
 -> EHLO yourserver.hosting.com
<-  250-smtp.example.com
<-  250-SIZE 5242880
<-  250-PIPELINING
<-  250-ENHANCEDSTATUSCODES
<-  250-8BITMIME
<-  250-DSN
<-  250-AUTH PLAIN LOGIN CRAM-MD5
<-  250 STARTTLS
 -> AUTH CRAM-MD5
<-  334 PDE4NTY1MzM3NDQuMTY4MTI5NTE2NkBzbXRwLmV4YW1wbGUuY29tPg==
 -> cGFzc3dvcmQ=
<-  235 2.0.0 OK
 -> MAIL FROM:<[email protected]>
<-  250 2.1.0 Ok
 -> RCPT TO:<[email protected]>
<-  250 2.1.0 Ok
 -> DATA
<-  354 Go ahead
 -> Date: Wed, 12 Apr 2023 10:26:06 +0000
 -> To: [email protected]
 -> From: [email protected]
 -> Subject: test Wed, 12 Apr 2023 10:26:06 +0000
 -> Message-Id: <[email protected]>
 -> X-Mailer: swaks v20190914.0 jetmore.org/john/code/swaks/
 -> 
 -> This is a test mailing
 -> 
 -> 
 -> .
<-  250 2.0.0 Ok: queued
 -> QUIT
<-  221 2.0.0 Bye
=== Connection closed with remote host.

The 250 2.0.0 Ok: queued response confirms the mail server accepted the message for delivery. Check the recipient inbox (including spam/junk) to verify end-to-end delivery.

Reading the SMTP transcript

The -> lines are what Swaks sent; the <- lines are server responses. Every <** line is a server-side rejection — the three-digit SMTP status code tells you what went wrong. For a full walkthrough of SMTP response codes and session structure, see How to read an SMTP session log.

What a successful Swaks test proves

A 250 Ok: queued response from Swaks confirms four things:

1. Network path: the WordPress server can reach the SMTP server on the configured hostname and port. No firewall, DNS, or routing issues.
2. Authentication: the username, password, and auth method are accepted.
3. TLS: if --tls was used, the server’s certificate is valid and the encryption negotiation succeeded.
4. Server accepts mail: the SMTP server queued the message for delivery.

One thing it does not prove: that the message reaches the recipient’s inbox. The SMTP server accepted the message, but it may still be rejected downstream (spam filters, relay configuration, DNS-based reputation checks). If Swaks succeeds but mail never arrives, the problem is on the mail server’s outbound relay side — contact the provider.

If Swaks succeeds and mail arrives, the SMTP layer is clean. Any remaining WordPress email problems are in the plugin stack: the mailer plugin configuration, the contact form, or wp_mail() itself.

For testing the WordPress side of the chain (once you have confirmed the SMTP server works), see How to test WordPress email with Mailtrap.

Troubleshooting: when Swaks fails

If Swaks cannot complete the SMTP transaction, the fault is between the WordPress server and the mail server — not in WordPress itself. Common failure points:

Symptom	Likely cause	Fix
Connection timeout / "Network is unreachable"	Wrong hostname or port; firewall blocking outbound SMTP	Verify hostname and port. Many shared hosts block port 25 — try 587 (submission) or 465 (implicit TLS). Contact the host if all SMTP ports are blocked.
530 Authentication required or 535 Authentication failed	Wrong username, password, or auth method	Double-check credentials. Match --auth to a method listed in the server’s EHLO response.
TLS handshake failure	Port/encryption mismatch; expired certificate	Port 587 typically uses STARTTLS (--tls). Port 465 uses implicit TLS (--tlsc). Mixing them up fails silently.
550 Relay denied or 553 Sender address rejected	The --from address is not authorised on this server	Use the exact sender address configured in the provider’s dashboard.

If the Swaks test fails, the mail server (or the network path to it) is the problem. Fix it at that layer before returning to debug WordPress plugins.