Due to changes introduced by Google regarding the use of external SMTP servers with Gmail accounts, you may occasionally encounter an error similar to:
TLS Negotiation failed
or a message indicating that Gmail could not verify the mail server’s security certificate.
Table of Contents
Cause of the Problem
This error typically occurs when Gmail cannot establish a secure TLS connection with the outgoing mail server configured for the account.
Common causes include:
- Incorrect SMTP server settings
- An incorrect SMTP port
- An unsupported encryption method
- Temporary connectivity issues
- Certificate validation problems
Verify Gmail SMTP Settings
To check your outgoing mail server configuration, open Gmail settings:
https://mail.google.com/mail/u/0/#settings/accounts
Navigate to:
Settings → Accounts and Import → Send mail as
Locate the affected email account and click:
Edit info
or
Edit SMTP server
Recommended SMTP Configuration
Make sure the following settings are configured correctly:
| Setting | Value |
|---|---|
| SMTP Server | smtp.mybox.com |
| Port | 587 |
| Connection Security | TLS |
| Username | Full email address |
| Password | Mailbox password |
Example Configuration
SMTP Server: smtp.mybox.com
Port: 587
Security: TLS
Username: [email protected]
Password: mailbox password
Important Notes
- Do not use SSL when port 587 is selected.
- Ensure that the full email address is entered as the username.
- Verify that the mailbox password is correct.
- If you recently changed DNS, MX, or SSL-related settings, allow time for propagation.
If the Problem Persists
If Gmail still reports a TLS negotiation or certificate verification error:
- Remove the external account from Gmail.
- Re-add the account using the correct SMTP settings.
- Verify that the mail server certificate is valid and up to date.
- Test sending mail using a desktop email client such as Outlook or Thunderbird.
- Contact your hosting provider to verify that the SMTP service is operating correctly.
Summary
When Gmail reports a TLS Negotiation Failed or certificate verification error, first verify that the outgoing mail server is configured as:
Server: smtp.mybox.com
Port: 587
Encryption: TLS
In most cases, correcting the SMTP server, port, or encryption settings resolves the issue and allows Gmail to send messages successfully through the external mailbox.