Problems with Spamhaus rejecting sent emails

[#1]

I'm unable to send emails due to the mailserver (91.103.217.10) being listed on Spamhaus lists.
Typical messages :
    SMTP error from remote mail server after initial connection:
    host mx1.bt.mail.yahoo.com [212.82.111.207]: 553 Mail from 91.103.217.10 not allowed - 5.7.1 [BL23] Connections not accepted from IP addresses on Spamhaus XBL
SMTP error from remote mail server after RCPT TO:<removed>:
    host mx.avasin.plus.net [212.159.9.200]: 550 IP is DNSBL listed - http://www.spamhaus.org/query/bl?ip=91.103.217.10
Is 91.103.217.10 a Plusnet mail server?
Adam
ps I can send via my gmail account, so not a burning issue.

[#2]

Ignore this - the domain appears to be a mail server used by one of my hosted email addresses. - nothing to do with PN
Adam