You've set up WP Mail SMTP, clicked Send Email on the Email Test tab, and got an error instead of a success message. Or the test says it was sent, but nothing arrives.
The error message usually tells you exactly what's wrong, once you know how to read it. This guide covers the errors people hit most often and how to fix each one. Most of the same advice applies to FluentSMTP and Post SMTP too.
First: where to find the details
When a test fails, WP Mail SMTP shows the error under the test result, often with a suggested fix. Read the whole message, not just the first line: the useful part is often near the end.
Newer versions of WP Mail SMTP can also record failed emails under WP Mail SMTP → Tools → Debug Events, which helps when the problem only happens sometimes.
"Could not authenticate" or "Username and Password not accepted"
The mail server rejected your login details.
Common causes and fixes:
- Wrong username or password. Re-enter them carefully. For most SMTP services the username is your full email address or a username the provider gives you, not your WordPress login.
- Gmail or Google Workspace with your normal password. Google doesn't accept your regular password for SMTP. Use the Google / Gmail mailer in WP Mail SMTP (which connects with your Google account), or create an app password if you use the "Other SMTP" mailer.
- Microsoft 365 / Outlook. Microsoft has been turning off basic password login for SMTP. Use the Microsoft 365 / Outlook mailer instead of "Other SMTP".
- API key pasted wrong. If you use an API mailer such as Brevo, Mailgun or SendGrid, check you copied the whole key with no spaces, and that it has permission to send email.
"Could not connect to SMTP host" or "Connection timed out"
WordPress couldn't reach the mail server at all.
Common causes and fixes:
- Your host blocks outgoing SMTP ports. Many hosts block port 25, and some block 465 and 587 too, to stop spam from hacked sites. Try port 587 with TLS, then 465 with SSL. If both time out, ask your host whether outgoing SMTP is allowed.
- Wrong host name or port. Check the SMTP host (for example
smtp-relay.brevo.com) and port against your provider's documentation. - Encryption doesn't match the port. Port 587 normally uses TLS, and port 465 uses SSL. Mixing them up causes connection errors.
The easiest fix: switch from the "Other SMTP" mailer to your provider's API mailer (Brevo, Mailgun, SendGrid, Postmark, SMTP.com and others are built in). API mailers send over normal HTTPS, so blocked SMTP ports stop mattering.
"Sender not verified", "From address not allowed" or "Domain not verified"
You connected successfully, but your email provider refused to send from the address you used.
Email services only let you send from addresses or domains you've proven you own. The fix:
- In your email provider's dashboard, verify your sending domain. You'll add DNS records (usually SPF and DKIM) at your registrar or Cloudflare.
- In WP Mail SMTP, set the From Email to an address on that verified domain.
- Tick Force From Email so every plugin on your site uses that address, not just WordPress itself.
Verifying the domain also improves deliverability, because it sets up DKIM signing. See SPF, DKIM and DMARC for WordPress.
"Your account is not activated" or sending limit errors
Some providers review new accounts before they can send, or put free accounts on a daily limit. Check your provider's dashboard for a pending review or an exhausted daily quota. Brevo, Mailgun and SendGrid all show this on their account pages.
The test says "sent", but nothing arrives
A success message means WP Mail SMTP handed the email to your provider. It doesn't guarantee delivery to the inbox.
Check these in order:
- Your spam folder, and any "Promotions" or "Other" tabs.
- Your provider's activity log. Brevo, Mailgun and others show whether the email was delivered, bounced or blocked, and why.
- Your authentication. If SPF, DKIM or DMARC fail, the receiving server may filter the email. The fastest way to check is to send the WP Mail SMTP test email to a free WP Mail Tester address: paste it into the "Send To" box on the Email Test tab, and you'll see every check for that exact email within a minute.
The mailer says "Default (none)" or "PHP"
If WP Mail SMTP's mailer is set to the default PHP option, the plugin is installed but not actually sending through an email service. Your site is still using PHP mail(). Choose a real mailer and connect it. Our guide to PHP mail vs SMTP explains why this matters.
Common questions
Should I use "Other SMTP" or a provider's API mailer?
Use the provider's built-in mailer whenever it exists. It's easier to set up, avoids blocked ports, and is usually more reliable. "Other SMTP" is for providers WP Mail SMTP doesn't support directly.
The test works, but my form emails still don't arrive. Why?
Your form plugin may be setting its own From address, often the visitor's email. Turn on Force From Email in WP Mail SMTP, and set your form's From address to your own domain. Our guide to contact form emails not arriving walks through each form plugin.
Do FluentSMTP and Post SMTP have the same problems?
Yes. They're different plugins doing the same job, so the same causes apply: login details, blocked ports, unverified sending domains and missing DNS records. Each has its own test email screen and logs.