A friend recently told me that she couldn't reply to some of my emails because they were bounced back to her as Spam, but she could send a reply (that reached me) if she first deleted the part that was my original email to her. 

We wondered if it had been blocked because my original message had contained lots of links, so could have been wrongly identified as Spam.

Now, today, I've had a problem with sending a reply to someone else, where my email is being blocked with Error_Code 552: Spam Message rejected. 

I don't know if this is relevant but I tried typing my IP Address into Spamhaus to see if it was on a blacklist and found it was included in their PBL (Policy Block List) but it said that this is usually something to do with SMTL Authentication settings.  

Is there anything that you or I can do to stop my emails being identified as Spam and blocked?  Thanks.