The below quote from a google search may apply to me.
My ISP when away is orange. fr I use outlook and my emails will not send but receive is OK - anyone know the setting to use?
For some reason, various ISPs (Orange, BT, AOL & a few others) block traffic on port 25 going to any SMTP server apart from their own, this is often a problem as other ISPs block all e-mail from certain other ISPs*.
Most SMTP servers will accept traffic on another port apart from 25 as a way of getting round this restriction, the number of this extra port varies from host to host, if they all used the same extra port ISPs would just block that one as well as port 25. You need to contact your host & ask if they have an extra SMTP server port.
Welcome to our forum, yes as you say in an effort to combat spamming ISPs will not allow you to send from port 25 with no authentication when not using their email addresses, luckily for our email addresses you can use port 465 with a secure login. Here are the details Email Settings for all Virgin Media domains please remember to configure the full email address as the username login.