The bounce back should explain the reason, such as spam looking or something else.
You may want to advise the person to change the subject line to something simple and also tell them to be careful what they put in it.
Different servers filter in different ways but you'll find most if not all filter content in the subject line, text in the body, and look at headers.
Enter your domain in here to do a blacklist lookup, just to be sure your not blacklisted anywhere.
http://www.mxtoolbox.com/blacklists.aspx
This is most likely not the issue but check to make sure you have a valid reverse lookup entry.
Some servers filtering software will bounce email if they do a reverse lookup on the email and find the sending server isn't the one that they did the lookup on.
What I mean is that if you send an email from your domain (abc.com), your email will then go to your ISP, which is then forwarded to the destination server.
If the reverse lookup is pointing at the ISP's mail server, the destination server will it treats it as a spoofed email and will bounce it, because the sending address is (mail.abc.com) but the reverse lookup is (ISP's server address).
