Many ISPs block outgoing email on port 25.Remove Thunderbird from your program's list of trusted or recognized programs, then add it back manually or when prompted to resolve this.Note: Firewall or antivirus software blocking outgoing email is a common cause of not being able to send messages after updating Thunderbird, since it may be set to trust the previous version of Thunderbird and no longer recognize the updated version as trusted. Try briefly turning both antivirus and firewall software off, sending a test email, and turning them on and sending another test email.Verify that your outgoing email is not being blocked by your firewall, antivirus software or your Internet Service Provider: Verify that your outgoing email is not being blocked For example, you cannot send a mail from through Yahoo's SMTP server. Most of the time you cannot send messages from one account provider using another account provider's service. Verify that you are using the right SMTP server for the email address. In the Thunderbird menu bar, click the Tools Edit menu and select Account Settings, or click the Application menu button and select Options Preferences and Account Settings from the sub-menu, and select the account name.

Verify the correct SMTP Server is being used Check that your Thunderbird settings match the documented settings. This is usually in the support section of their website searching for 'mail settings' or 'SMTP' will usually find it. Look for an SMTP mail settings article on the mail provider's site.

Check that the settings are the same as documented in the article for your mail provider ( Gmail, Yahoo, Comcast, Hotmail, Sympatico) OR

In the Thunderbird menu bar, click the Tools Edit menu and select Account Settings, or click the Application menu button and select Options Preferences and Account Settings from the sub-menu, and select Outgoing Server (SMTP) from the bottom of the list in the left panel.

Verify that your mail server supports the minimum version of TLS.

Verify that your outgoing email is not being blocked.

If your existing configuration suddenly stopped working after updating to Thunderbird 78, it might be your Antivirus or Firewall blocking the new version, or your email provider using outdated TLS security protocols. The problem suddenly started after updating to Thunderbird version 78?

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