I know one of the Wanadoo servers was on the spamcop blocklist the other day, check your senders IP at
http://www.spamcop.net/bl.shtml
If it's listed you know the cause and can bring it up with them.
Someone posted a freephone number on one of the forums,
0800 183 0213 press 3 when prompted and it takes you to the normal line.
Not tried it yet, but next time...much better than paying for the call !!