Internet Service Providers (or "ISPs") have recently begun to block port 25 for outgoing mail. This is the port that programs like Outlook, Mac Mail and Thunderbird use to send email by default.
ISPs such as Orange, O2 and TalkTalk block this port because they want you to send through their outgoing server. This means that they can do things like stop spam and so on. It’s possibly a way for ISPs to regulate your usage for the budget price they charge.
We have a work around which lets you avoid this block. You can set your outgoing mail port to 26. This should bypass the block.
Update: If Port 26 doesn’t work either
Some ISPs now also block 26. The only additional way to get around this error is to enable SSL on your email connection and set the port to 465. If you don’t have an SSL on your domain then you can also use your server hostname.