The most common reasons not to be able to send emails are:
* Incorrect Thunderbird settings;
* Blocked port 25 by the Internet Service provider.
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, since SiteGround servers require such authentication.
2. Try using the alternative TCP port 2525 for the SMTP server. For more information you can check our Thunderbird tutorial.
If you are having problems with your email, you may contact your host for assistance. SiteGround provides the best email hosting and we will assist you with any email issues you may have.