WP Mail SMTP fixes WordPress email by sending it through a real mail account or service instead of the web server. When it stops working, the plugin usually tells you why. The trick is reading the error.
New to this? Start with why WordPress email fails in the first place: How to fix WordPress not sending email.
1. Send a test and read the debug output
Open WP Mail SMTP’s Email Test tab (under WP Mail SMTP → Tools in recent versions) and send a test to an address you can check. If it fails, the plugin shows debug output with the mail server’s actual response. The free version also records failures under Debug Events.
2. Match the error
| Error | What it means | Fix |
|---|---|---|
| SMTP Error: Could not authenticate | The username or password was refused. | Re-enter the credentials. Gmail and Microsoft 365 usually need an app password or OAuth instead of your normal password, and some Microsoft 365 tenants have SMTP sign-in turned off. |
| Could not connect to SMTP host / Connection timed out | Your server could not reach the mail server at all. | Check the host name and port. Many hosts block outgoing SMTP ports; try port 587 with TLS, or switch to a mailer that sends over HTTPS (an API mailer). |
| SSL or certificate errors | Encryption setting does not match the port. | Use TLS with port 587, or SSL with port 465. Do not mix them. |
| invalid_grant (Gmail / Google Workspace mailer) | Google revoked or expired the connection. | Reconnect the account. If it keeps happening every week, your Google Cloud app is still in Testing status. |
| 550 5.7.x … sender not allowed / not verified | The From address is not allowed by your mail provider. | Turn on “Force From Email” and use an address your provider has verified. |
| Limits reached / quota exceeded | You hit the account’s daily sending limit. | Gmail and Microsoft accounts have daily limits. Use a sending service built for website email. |
3. Check for conflicts
- Deactivate any second SMTP, mail-logging or “email delivery” plugin. Only one can control WordPress mail.
- Some themes and form plugins have their own SMTP settings. Turn those off so WP Mail SMTP is the only sender.
- If the test works but forms do not, a form is setting its own From address. Turn on “Force From Email”.
Using your personal Gmail or Outlook account as the mailer ties your website to that account’s password, security settings and daily limits. A password change or security alert can stop all site email without warning.
4. Mail is sent but lands in spam
If the test succeeds but messages go to spam, the From domain is not properly authenticated for the service you send through. Make sure the service is in your SPF record and signs with DKIM for your domain. See SPF vs DKIM vs DMARC.
An alternative without SMTP credentials
SecureSMTP’s WordPress plugin replaces the whole setup: no SMTP host, port, password or Google Cloud app. You paste one API key, and every email your site sends is DKIM-signed and listed in a delivery log as delivered or bounced. See how it compares with WP Mail SMTP.
Frequently asked questions
Why did WP Mail SMTP stop working after a week with Gmail?
If the Google Cloud app you created for the Gmail mailer is still in “Testing” status, Google expires its access after about seven days. Publish the app (set it to “In production”) and reconnect, or use a mailer that does not depend on your Gmail account.
Can I run two SMTP plugins at the same time?
No. Only one plugin can take over WordPress’s mail function. Two SMTP or mail-logging plugins fighting over it is a common cause of failures. Keep one.
The test email works but my contact form emails do not. Why?
The form probably sets its own From address, often the visitor’s email, which your mail provider then rejects. Turn on WP Mail SMTP’s option to force the From email, or set the form’s From address to your own domain.