I moved Thunderbird from my old MacBook Pro to a new MacBook Pro (I have done this before with no problems). I updated Thunderbird on both laptops before I did this - to version 31.0 The new machine's running Mavericks 10.9.4 I started to notice intermittent problems with sending email - this seemed to free up when I opened up my BT email account (the account that 'feeds' Thunderbird) and then resent the email in Thunderbird. This 'fix' has now stopped working and all I continually get when trying to send is the message: 'Send Message Error The message could not be sent because connecting to SMTP server mail.btinternet.com failed. The server may be unavailable or is refusing SMTP connections.' (etc.) I checked with both Mozilla and BT forums/help pages, then went to Thunderbird>Tools>Account settings>Outgoing Server(SMTP)>Edit I then changed the settings to those recommended by both BT and Mozilla: Server Name: mail.btinternet.com Port - 465 Security - SSL/TLS Authentication - normal password User Name - (my user name) I also tried many variations (including the original settings) Server Name: mail.btinternet.com Port - 587 Security - none Authentication - password transmitted insecurely User Name - (my user name) I tried deleting my password and then doing the above all again. I tried turning the router off and back on and doing the above all again. Nothing - apart from not being able to send email Thunderbird's working fine with incoming, I really think I've tried everything I can. Can I delete my prefs - will this help? Is there anything I've missed? I've wasted 4 hours and it feels like 4 days! Any help would be greatly appreciated.