[Quoth]

Some UK ISPs only allow their SMTP server and block SMTP to any other Mail Exchange / Server / Relay.
Some ISPs that offer SMTP server only allow requests from their own IP addresses, so when you leave home or office and are on a different ISP, you can't send email.
Some email systems may block the entire IP range of an ISP if that range has a lot of spammers. Sometimes they block an IP range they think is in that and isn't!