The most common reasons not to be able to send emails are:
* Incorrect Email Client settings;
* Blocked port 25 by the Internet Service provider.
* You have reached the send limit
To resolve the problem you should try to:
1. Make sure you are using the correct Thunderbird settings:
* Incoming mail server (POP3/IMAP server): mail.yourdomain.com
* Outgoing mail server (SMTP server): mail.yourdomain.com
* Username: the full e-mail address like firstname.lastname@example.org
* SMTP authentication must be enabled for the login to be successful.
A common mistake is to leave "My SMTP server requires authentication checkbox" unchecked. This option should be always enabled, our servers require such authentication.
2. Try using the alternative TCP port 587 for the SMTP server.