Thunderbird needs the correct settings to send and the settings are setup by iprimus - Thunderbird does not choose whatever settings are required and it is not a server. But I think you are talking about trying to use a method of relay via another server to send. Generally the iprimus SMTP server settings would be: * Outgoing Server SMTP: smtp.iprimus.com.au * Port: 25 * Connection Security: None * Authentication Method: Password transmitted insecurely * User Name: full email address Did you previously select 'Authentication Method' to send password insecurely? Or did you select 'No authentication' ? If the latter then try using 'Password transmitted insecurely'. However, you may find the residential home ISP is blocking port 25 for security reasons. This is the ISP not allowing sent via port 25 as a relay for other servers. It's also common for most public wifi hotspots to block port 25 as well. It's a common practice among ISPs to prevent spam and other email-related security issues and most commonly used by hotels etc. Usually, you can try the following to see if it gets allowed but I did not find these settings mentioned by iprimus. * Outgoing Server SMTP: smtp.iprimus.com.au * Port: 587 * Connection Security: STARTTLS * Authentication Method: Normal Password * User Name: full email address In addition make sure the computer Firewall (note if installed Anti-Virus then it may control the Firewall) has Thunderbird program set up as an allowed program. An alternative you could try would be to create a gmail account and get emails forwarded from iprimus account to the gmail account, then set up gmail account in Thunderbird.