Spam filters blocking outgoing mail

[#1]

I have recently had trouible sending email to a recipient whose spam filter appears to be blocking emails from me.  I received the following message :
... while talking to gr.mx0.global.net.uk.:
<<< 550-Host 212.100.250.226 is rejected as host is blocked by our real-time black
<<< 550 list (see http://www.spamhaus.org/SBL/sbl.lasso?
What concerns me is that several other mails over the last few weeks have disappeared into the blue which I can only put down to spam filtration.
It does not seem to make any difference whether I use Outlook or SquirrelMail, because there have been incidents on both.  The emails have all been fairly innocuous, and the only common denominator has been that they have all emanated from the Plusnet servers of course.  I sometimes use other email platforms like UK2.net and Tiscali.co.uk and Tiscali.fr.  I have never experienced any similar problems with any mail using their servers.  However, the amount of use is an unfair comparison with Plusnet, but it is nevertheless an observation worth mentioning.
Has anyone else experienced similar problems ?  Is there any reason to suspect that Plusnet mail may be filtered out by various spam filters ?  Obviously, it could be to do with me, of course, but the mail in question has all been innocuous, and often just replies to thrid parties effected by hitting the Rreply button.
Most strange.....

[#2]

It's the IP address 212.100.250.226 which is being blocked which a rDNS give phosphorus.onspeed.com. Do you recognise this?

[#3]

212.100.250.226 is listed here www.spamhaus.org/sbl/sbl.lasso?query=SBL59503

[#4]

Thanks for that.  It explains it all !
OnSpeed is an accelerator which compresses any downloads and expands them automatically on the pc - ideal for travelling when having to use dial-up.  It increases speeds by three or four times, and it even works on broadband, though the difference is less obvious.  But it means all traffic goes through their servers.
There, I am going to have to turn it off and use it more selectively.
Thanks for that.

[#5]

Haven't used Onspeed but I am familiar with it.
That would have been my next post....honest

[#6]

Just out of interest does anyone know if OnSpeed operates on all traffic or just inbound.
Logic would suggest it only operates on inbound and therefore outbound traffic such as mail would not be affected and would not carry OnSpeed IP address.