I can't believe I'm the only person this has happened to, but a few days ago I lost the ability to send emails. When you try to send it ticks away for a while and then times out. I hadn't changed any settings, it just stopped working. I useThunderbird but had the same problem on another PC running Outlook. I wasted the best part of a day yesterday trying to sort it out, including a few fruitless hours on the phone to BT support where they remotely accessed my computer without any success. Eventually I was put to a higher level of IT support (I was warned that I may be charged for this) I described my situation:
I have a non BT domain with an email address not supplied by BT. My incoming mail comes via the non BT provider. So my POP settings in Thunderbird (or Outlook) are Server type: POP mail server, Server name: mail.something.co.uk, User name: xxx@something.co.uk. For SMTP settings I had been using mail.btconnect.com on port 25.
I eventually found out that in order for outgoing to work with this configuration, BT (or whoever operates your broadband) have to provide something called a mail relay server, this, I am told is old technology and encourages spammers, so ISP's are switching them off. This is what BT has done and why sending no longer works. They said I should have been told about this, but I wasn't. However, fortunately it's easy to resolve.
What you do is change your SMTP settings to the same as your POP settings. Theres a box to tick in Outlook to do it automatically. I am on version 2007 but later versions are probably similar:
Tools
Account Settings
highlight the email address in question
Change
More Settings
Outgoing Server
Tick the box "My outgoing server (SMTP) requires authentication
select "Use the same settings as my incoming mail server"
And that should do it
In Thunderbird
Tools
Account settings
Outgoing server (SMTP)
highlight the relevant email address and click Edit
you put the mail server info in (eg. mail.something.co.uk) Port 25 Connection Security: None, Authentication Method: Password sent insecurely. Username eg. xxx@something.co.uk. When you first try and send an email (Thunderbird) it will prompt you for a password which is the same as the one in your POP setting and you can save it so you don't have to do it every time.
There are probably other ways but this worked for me.