Emails sent from mobile to plusnet email accounts wrongly labelled as spam

[#1]

I have been trying to send emails from my mobile phone, from my own website email address to plusnet email a/cs, but these are all blocked as Junk Mail with error code 550. The reference to Spamhaus. Anyone got any idea why this is happening and how to stop it?

[#2]

What are you using as the outgoing mail server address?

[#3]

Just Googled for error code 550 and got this: http://www.gordano.com/kb.htm?q=475
Not sure if that's anything to do with it, but myself I can send and recieve emails on my mobile via my ISPs smtp server, no problems.