In reply to a post by XRaySpeX:

Note: We are only talking here where Orange is the home ISP, where port 587 is appropriate home & away. Other home ISPs may need a diff port or not have one at all.

Including O2's authenticated smtp server, which only listens on port 25. I always found this incredible given that many of their (mobile) customers are likely to be using a diverse range of networks to send mail over, many of which may block port 25 outgoing. What's perhaps even more incredible is that it doesn't have SSL either.